1. Preheat the oven to 180°C (350°F). Line three springform tins (18cm)* with baking parchment and grease. Set aside.

2. In a mixing bowl combine the butter with sugars and beat on high speed until well combined – for about 2-3 minutes. Add the eggs and mix for another 2-3 minutes until all is well combined and creamy. Reduce the speed and gradually add the buttermilk, oil and vanilla extract. Increase the speed and mix until you get a smooth and fluffy mixture – it will be quite runny at the beginning and the fat is probably not willing to come together with the liquid, but keep mixing and you will get a smooth batter – takes up to 10 minutes or more. When the batter is smooth and there are no more streaks of fat or liquid, reduce the speed and add the flour, baking powder and salt (mix it in a bowl before adding) – mix until just combined. Add the rainbow sprinkles and fold in with a spatula. Fill 1/3 of the batter in each of the springform tins and bake for about 30-38 minutes or until a toothpick inserted in the center comes out clean. Take out of the oven, loosen the cake with a knife from the tin and let cool down completely on a wire rack. Wrap in plastic wrap and store in the fridge overnight.

3. Preheat the oven to 180°C (350°F). Line a baking sheet with baking parchment and set aside. In a bowl mix sugars, flour, baking powder, salt and the rainbow sprinkles until well combined. Add the oil and vanilla extract and mix with a fork until streusels form. Make sure there are different sizes of streusel. Transfer to the baking sheet and bake for about 15 minutes. Take out of the oven and let cool down completely. Store in an airtight container if needed (up to 1 week) in the fridge.

4. For the frosting add the butter and cream cheese to a mixing bowl and cream together for 2-3 minutes on medium speed. The mixture should be light and fluffy. Gradually add corn syrup (optional) and vanilla extract and beat until the mixture is white and smooth – about 3-4 minutes. Add the confectioner’s sugar, salt, baking powder and citric acid (optional) and mix first on low then on high speed until you have a nice white smooth frosting.

5. Bring the cake layers in shape by using an (adjustable) cake ring that is slightly smaller than the cake to cut out a circle – that way you get a nice edge. Do the same with the other layers. Eat the leftovers. Line the inside of the cake ring with some greaseproof paper or baking parchment and place the first cake layer in it – the paper should have the height of the finished cake (check the height of the cake layers combined). Mix the milk with the vanilla liquor and brush the cake with half of that mixture. Add 1/5 of the frosting and spread evenly on top of the cake (by using the back of a spoon for example). Sprinkle with 1/3 of the streusels and press them a bit into the frosting. Add 1/5 of the frosting and spread evenly so you get one even layer with the streusels inside. Add the second cake layer and repeat. Brush with the remaining milk-liquor mix, frosting, and streusels, frosting – then place the last cake layer on top and spread the remaining frosting evenly to get a smooth finish. Sprinkle with the remaining streusels – if you have big ones, place them in a circle. Place in the fridge for at least 6-8 hours.

6. Take out of the fridge at least 30 minutes before serving and remove the cake ring and paper. Smooth out the frosting on the sides if necessary. Serve with a candle on top and sing »Happy Birthday«.