To make the cake layers:

1. Using an electric mixer (ideally handheld, since the quantities are so small), 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 egg and vanilla, mixing on the lowest speed for about 30 seconds, until incorporated.
3. Add room temperature buttermilk and oil and mix on low speed to incorporate.
4. In a bowl sift flour, baking soda, baking powder and salt. Pour into 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.
5. Line 6 cups of a cupcake pan with wrappers and divide batter between them. I like to use a cupcake scoop for this! 
6. Bake at 175ºC or 350ºF for 15 minutes or until the cupcakes spring back up when you poke them.
7. Leave cupcakes to cool in the pan for 10 minutes before lifting them out.
8. Unwrap the cupcakes and cut each cupcake in half horizontally to make two layers. You'll need five cupcakes to make 10 cake layers, of which you'll use 9 to make three 3-layer mini cakes.

To make the buttercream:

1. Sift the sugar to remove any lumps and check that the butter is at room temperature - you should be able to slice through it easily with a spatula.
2. With an electric mixer (preferably handheld, since the quantities are too small to reach effectively with a stand mixer), mix butter for a few seconds until smooth. Add the powdered sugar and salt and mix on the lowest speed until incorporated, about two minutes.
3. Add vanilla and mix until it's combined with the buttercream.
4. To make flavoured fillings, scoop up about half a cup of buttercream and divide between three small bowls. Add 1/2 a tablespoon of any filling to each bowl (e.g. dulce de leche or caramel, crushed freeze dried strawberries, lemon curd, jam, or crushed Oreo cookies). Stir to check the consistency and if it's too stiff, add a few drops of milk until it's easy to stir. Note: if you're using liquid filling ingredients e.g. caramel, the consistency will probably be perfect without adding milk! For dry ingredients e.g. cookie crumbs or crushed freeze dried strawberries you'll probably need to add milk.
5. Check the consistency of your main bowl of frosting and add more milk or cream a tablespoon at a time until the buttercream is smooth and easily stirred.

To make a mini cake:

1. Spread or pipe a blob of buttercream onto the middle of a cake board at least 4 inches wide and press the first cake layer down onto it to attach it.
2. Add a layer of flavoured buttercream as filling, spreading it over the top of the cake layer.
3. Repeat with the next cake layer and filling and then place the final cake layer on top.
4. Put the cake into the freezer for 15 minutes or the fridge for 30 minutes to set the buttercream.
5. Cover the cake with a thin layer of buttercream as a crumb coat. This is easiest to do by piping it onto the cake (I like to do zig zags around the cake) and then spreading it. When it's fairly smooth put the cake back into the fridge or freezer for 15-20 minutes.
6. Decorate the mini cakes following the steps in the tutorial!