1. Measure out all ingredients you need and let them come to room temperature – this way you will get the best results.

2. Preheat the oven to 350°F (180°C). Line (and lightly grease) four 8-inch (20cm) cake pans with baking parchment and set aside. Silicone molds* work really well here and can be used without baking parchment, for example ;)

3. Add butter and sugar to a large bowl and mix on high for 6-7 minutes until very light and fluffy. Add the oil and vanilla extract and mix until well combined. Next, add the eggs one after another and mix each time until well combined. In a second bowl combine flour, baking powder, baking soda, and salt. Add this mixture in three batches, alternating with the buttermilk, to the bowl, and mix until just combined. Add the rainbow sprinkles and fold in. Divide the batter evenly between the cake pans, smooth out the tops, and bake the cake layers for about 25-26 minutes or until a wooden skewer inserted into the center of the cakes comes out clean (see note at the end). Take the cakes out of the oven and let them cool down inside the pans for some time, then remove and let cool down completely on a wire rack. The cake layers can be baked a day in advance – just make sure you wrap them airtight until you are ready to assemble the cake.

4. For the buttercream start by cooking a custard. Use about 3-4 tablespoons of the milk and mix it with cornstarch, sugar, and vanilla extract. Add the rest of the milk to a small saucepan and bring to a boil. Add the cornstarch mixture to the bubbling milk and stir in. While stirring constantly let the custard cook for a moment until thickened nicely. Remove from the heat and immediately place a piece of plastic wrap (cling film) on top of the custard to prevent skin from forming. Let it cool down completely. That is something you can do a day in advance as well. If the custard gets chunky I recommend mixing it well (e.g., with an immersion blender) before using it in the buttercream.

5. To finish the buttercream add the butter and confectioners’ sugar to a large bowl and mix on high until very light and fluffy – my stand mixer needs 4-5 minutes for that. When the butter is fluffy, gradually add the custard (a spoonful each time) and mix until well combined. When all custard is incorporated, mix for several minutes longer to get a smooth and silky buttercream.

6. Before you assemble the cake, I recommend cutting off the edges of the cake layers. Use a round cutter that is slightly smaller than the cake layers and press it into the cakes to get perfectly round cake layers that are all the same size. Place the first cake layer on a serving plate and spread a small amount of buttercream on top. Press halved raspberries into the buttercream. Add the next cake layer on top and repeat the process with the buttercream and raspberries. Finish with a last cake layer and then use the remaining buttercream to completely cover the cake – top and sides. If you want the cake to look neat, you should smooth out the buttercream around the cake, but little waves here and there are also fine. Decorate the cake with some additional rainbow sprinkles to your liking.