To bake the cake:

1. Using a mixer with a beater/paddle attachment (not a whisk), beat room temperature butter and sugar on medium speed until pale and creamy, about 2 minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
2. Add eggs one at a time, mixing on the lowest speed after each addition for 30 seconds. Scrape down to the bottom of the bowl to make sure everything is incorporated before adding the next egg.
3. Add vanilla and mix for about 30 seconds to combine.
4. Add room temperature buttermilk and oil and mix on lowest speed to incorporate.
5. In a bowl, sift flour, baking soda, baking powder and salt. Add to the mixing bowl and mix on lowest speed to incorporate, scraping down to the bottom of the mixing bowl to check that batter is evenly mixed.
6. Pour sprinkles into the batter and fold in gently with a spatula.
7. Grease three 6 inch baking pans with non-stick oil spray or butter. Divide batter equally between the cake pans.
8. Bake at 160ºC or 325ºF for 30 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
9. Leave cakes to cool in their pans for 10 minutes. Use a spatula to loosen the cakes from the edges of the pans and then turn onto a wire cooling rack. Leave to cool completely.

To make the buttercream:

1. Mix butter and sugar together at low speed for 4 minutes, scraping down to the bottom of a the mixing bowl a few times, until smooth. Add vanilla and salt and mix to combine.
2. Add milk 1 tablespoon at a time until the buttercream is the consistency of peanut butter or stiff whipped cream. 
3. Optionally, tint the buttercream using food colouring.

To put the cake together:

1. Spread a bit of buttercream onto the middle of a cake board or flat plate or platter. Press the first cake layer onto the buttercream to secure it in place.
2. Spread about 1/3 cup buttercream to cover the top of the cake layer and then place the next cake layer on top. Spread another 1/3 cup buttercream on top and then add the final cake layer. If you have time, put the cake into the fridge for 30 minutes or the freezer for 15 minutes before frosting it.
3. Spread half of the remaining buttercream to cover the cake with a crumb coat. Smooth with a cake comb or offset spatula. Let this set in the fridge or freezer for 15-30 minutes.
4. Set aside about 1/2 a cup of the remaining buttercream and cover with plastic wrap for piping later. Stir sprinkles gently into the rest of the buttercream.
5. When the crumb coat has set, spread sprinkled buttercream over the cake and scrape around a few times with a cake comb. Spread more buttercream over any indents or shallow areas or trails caused by the sprinkles and scrape around the cake with your cake comb again. Repeat until the frosting is smooth. Flatten the top edge by swiping sideways with an offset spatula or angles palette knife.
6. Chill the cake for 15-30 minutes in the fridge or freezer before piping a border onto the top. This prevents the weight of the piping causing bulging or drooping frosting.
7. Serve this cake at room temperature. You can refrigerate it for 2-3 days but take it out of the fridge at least 2 hours before serving.