In this tutorial I’m going to show you how to decorate this cheerful striped rainbow cake with a rainbow drip! If you prefer to watch a video of this tutorial, scroll down to the bottom of the page.
Cover your cake with a crumb coat of frosting to trap in any crumbs that might come off the cake. It doesn’t need to be perfectly smooth and it’s fine if you can see the cake through the frosting because we’re going to completely cover it up.
Although it doesn’t need to be perfect, the crumb coat should give the cake the shape you want, with straight sides and a level top. Let the crumb coat set for at least an hour at room temperature. I put mine in the fridge for 30 minutes.
Press a striped cake comb against the side of the cake in a few places to imprint the grooves of the comb, so you know where the stripes will eventually be on the cake. Each little line indented in the cake will be a different colored stripe at the end.
Use these imprinted lines as a guide to spread your colours of buttercream around the cake, making sure you have a different colour going over each of the lines. I’m using my 4 Minute Buttercream and I used gel colours to tint small bowls to make 9 different rainbow shades.
You could pipe the buttercream on instead of spreading it, but since I have 9 colours I’d need 9 piping bags and that’s a lot of washing to do afterwards, so I’m using my offset spatula to spread the colours on instead.
When you’ve covered the whole cake in bands of colored buttercream, smooth the frosting by scraping around the cake a few times with a frosting smoother. If you’re looking for variations on striped designs, check out my tutorial on 4 Secrets for Perfect Stripes on Cakes.
Wherever there are air pockets in the frosting, spread on some more buttercream of that colour to fill in the gap. This rainbow frosting doesn’t have to be perfectly smooth but the smoother it is, the fewer touch-ups you’ll have to do after using the striped cake comb.
Scrape around the cake again to smooth the frosting and when the frosting is not perfect but fairly smooth, switch to your striped cake comb. Rest the base of the comb on the cake board and press it gently against the side of the cake and then spin the turntable to scrape all the way around the cake, leaving stripe grooves in the rainbow frosting on the cake.
Scrape any buttercream off your cake comb into an empty bowl and save this for piping the 1M swirls on top of the cake later.
After scraping around the cake a few times with your striped cake comb you’ll probably have several air pockets in the stripes. Fill them in by spreading on some more buttercream of that colour.
Scrape again and again with your striped cake comb, touching up whenever you notice air pockets, until the stripe grooves are clear and smooth.
Tidy up the top edge of the frosting with an offset spatula or frosting smoother, or you can leave this as it is and after chilling the cake you can cut it off with a sharp knife. Put the cake in the fridge for 30 minutes or in the freezer for 15 minutes to set the stripe grooves.
Meanwhile, fit a piping bag with a star shaped tip like a Wilton 1M tip. Now we’re going to use up the rainbow buttercream you scraped off the cake with your frosting smoother and striped cake comb.
I’m filling half of my bag with plain white buttercream and then other half with the blended rainbow buttercream but you can use only the rainbow buttercream if you like. Set this aside for some finishing touches at the end.
When the frosting on the cake is chilled and firm, take the cake out of the fridge and spread white buttercream all over the cake to fill in the stripe grooves. You’ll spread it all over the rainbow stripes too but that’s fine, we’ll tidy everything up in a minute. Spread quite firmly to push the buttercream all the way into the grooves so there aren’t any air pockets.
Cover the top of the cake as well and now it’s time to reveal the rainbow stripes! Scrape around the cake with your frosting smoother, which will start to smooth the frosting and take off some of the excess white buttercream.
Keep scraping again and again, which will take off a little bit more buttercream with each scrape. The rainbow stripes will be very subtle to start with as they begin to show through the frosting, and they’ll have lots of white patches within them, but with each scrape they’ll get clearer and neater.
If you pressed your striped cake comb into the crumb coat before you started and used those indents to guide where you spread or piped the colored buttercream, you’ll end up with a different colour for each stripe but the colours will fade through each other like a rainbow, blending together nicely.
Tidy up the top edge with an offset spatula or frosting smoother, scraping from the outside of the cake towards the middle and wiping your offset spatula or frosting smoother clean after each scrape so that you don’t drag excess buttercream back onto the sharp edges of your cake.
If you want to give the cake a sprinkled border around the bottom, it’s easiest to do this now before the frosting sets completely. Place the cake on a baking tray or hold it above a large cake pan and pick up handfuls of sprinkles, pressing them into the bottom inch or so of the cake.
The white frosting will be sticky because it’s still fresh and soft, but the coloured buttercream has been chilled so it won’t be sticky anymore so you’ll have to press quite firmly for the sprinkles to stick to it. Now put the cake back in the fridge to set the frosting again.
For the drip you’ll need lots of bowls. You can use candy melts or white chocolate for this. If you’re using candy melts, place a few coloured candy melts in each bowl, combining colours to make any shades you don’t have, like pink and blue to make purple.
Now you can either warm heavy whipping cream or double cream until bubbles form around the edges and then pour the cream into each bowl, or spoon cold cream into the bowls and put them in the microwave one by one. For drips made with candy melts you need 4 times the candy melts compared to cream, for example 40g of candy melts and 10g of cream.
After melting the drips and letting them come to room temperature, do a test drip on the side of the bowl before you move on to your cake. If you’re using white chocolate instead of candy melts, melt 3 parts of white chocolate with 1 part of cream, for example 300g chocolate with 100g cream, and then divide the drip between bowls and add a tiny bit of gel colour on a toothpick to each bowl.
Take your cake out of the fridge and apply the drips with a spoon, scooping up some of the drip and then holding your spoon above the edge fo the cake and angling it to let the drip spill out slowly, moving your spoon around the edge of the cake and pausing wherever you want there to be a drip.
Your cake needs to be cold for this because the coldness of the cake stops the drip from running all the way down to the bottom of the cake. Change colours as you work your way around the cake and ideally all of your drips will be made with the same ratio of cream to chocolate or candy melts, so they’ll all be the same consistency and you’ll get the same drip effect but in lots of different colours.
Don’t worry about how neat the edge of the drip is around the edge of the top of the cake because you’re going to cover that up in a moment.
When you finish the drip, pipe swirls or ruffles around the top of the cake with the leftover rainbow buttercream in a piping bag fitted with a Wilton 1M star piping tip. To pipe these ruffles I’m wiggling my bag back and forth so that the buttercream folds over itself.
Because I filled one side of the bag with white buttercream and the other half with rainbow buttercream, the ruffles are white on one side and rainbow on the other side. Hold your bag in the same position as you work your way around the cake so that the colours line themselves up the same way for each ruffle.
Scatter a few sprinkles on the top of the cake and press a few into the sides in random places, or if you have any smudges on your stripes you can be strategic about where you place the stars so that you cover those up.