1. Start with the buttercream by making a pudding. Add the cornstarch and sugar to a small bowl and mix with some of the milk until the cornstarch has dissolved completely. Add the rest of the milk and vanilla extract to a small pot and bring to a boil (stir constantly). As soon as the milk starts to bubble, add the cornstarch mixture and mix in until the pudding starts to thicken. Take off the heat, cover with a piece of plastic wrap and let cool down completely. Make sure the plastic sits directly on the surface of the pudding so you don’t get a “pudding skin”.

 

2. Preheat the oven to 350°F (175°C). Line or grease three 6 inches (15cm) springform tins and set aside.

 

3. Add the butter and sugar to a large bowl and mix until light and fluffy. Add the egg whites and vanilla extract and mix until fluffy and pale. Add the milk and sour cream and mix until well combined. Mix the flour with baking powder, baking soda, and salt and add to the large bowl – mix until just combined. Add the rainbow sprinkles and fold in – try not to move them too much or the color will bleed out a lot. Divide between the three baking tins and bake for 28-32 minutes or until a skewer inserted into the center comes out clean. Take out of the oven, let cool down a bit in the tins, then remove and let cool down on a wire rack.

 

4. As soon as the pudding has cooled enough and the cakes are cooled, you can finish the buttercream. Add the softened butter and confectioners‘ sugar to a large bowl and mix on high speed until light and fluffy. Add the cooled pudding gradually with a spoon while mixing constantly until you get a nice and smooth buttercream. Take about 1/4 of the buttercream and mix with the rainbow sprinkles – be gentle and do not mix a lot or the sprinkles will bleed out and you end up with a weird colored buttercream.

 

5. Place the first cake layer on a serving plate and spread half of that sprinkle buttercream on top. Place the second cake layer on top. Spread the remaining sprinkle buttercream on that layer and finish with the third cake. Make sure everything is even and straight and place the cake for 20-30 minutes in the fridge. As soon as the cake has cooled enough (if the edges are wonky cut them straight) coat the whole cake with a thin layer (crumb coat) of the white buttercream. Place the cake once more in the fridge for 20-30 minutes, then coat completely with a thick layer of buttercream – make sure to keep some for the decorations. Smooth out the top and edges and place the cake back in the fridge for another 30-60 minutes.

 

6. Color the remaining buttercream in the colors you want and add a bit randomly on the top and sides of the cake – smooth out again so you get some soft colors all around. Use a small spatula or the back of a small spoon and spread dots of color all over the cake. Let the finished cake cool some more before serving.