1. Preheat the oven to 350°F (180°C). Lightly grease four 6 inches (15cm) baking tins (I used silicone molds) and set aside. Make sure your ingredients are all at room temperature. Melt the butter and let cool down a bit.

2. Add the flour, sugar, baking soda, and salt to a large bowl and mix until well combined. In a second bowl mix the eggs with buttermilk, melted butter, oil, and vanilla bean paste until well combined. Add the wet ingredients to the bowl with dry ingredients and mix until well combined. Divide the batter between the prepared baking tins, smooth out the top, and bake for about 27-30 minutes or until a toothpick inserted into the center of the cakes comes out clean. Take out of the oven and remove the cakes immediately from the tins and let cool down on a wire rack.
Note: You can bake the cakes a day in advance, wrap them with some plastic wrap when cooled and leave them on the counter until you continue with the cake.

3. For the buttercream add the egg whites, sugar, and salt to the bowl of your kitchen machine (with heating function) or place it on a pot with simmering water – the water should not touch the bowl in this case! Mix until the sugar has dissolved completely and the mixture has reached a temperature of 160°F (71°C). It’s best to use a kitchen thermometer here to check the temperature. Once the temperature is reached, turn off the heating function or remove the bowl from the stove and transfer the mixture to the bowl of a stand mixer. Whisk on high for about 5-8 minutes until stiff peaks form and the mixture has cooled down to room temperature, more or less.

4. Gradually add the soft butter (about a tablespoon at a time) and mix well – only add more butter when the butter you just added has disappeared. Add the vanilla bean paste and confectioners’ sugar and mix in slowly, then mix on high for another 5-6 minutes until you got a very soft and silky buttercream.

5. To assemble the cake, first straighten the cake layers, if they are domed, then cut out a hole in the center of three of the cake layers with a round cookie cutter (about 1.5-2 inches in diameter) – this is where the filling will go in later. Place the first cake layer (with hole) on a serving plate and spread some buttercream on top. Place the next cake layer (with hole) and repeat the process, same with the third cake layer. Fill the cavity in the center with smarties and rainbow sprinkles (optional), then place the last cake layer (without a hole) on top to close the filled space. Spread a thin layer of buttercream over the entire cake (doesn’t have to look super clean) and place it in the fridge for about 20-30 minutes.

6. Apply another layer of buttercream – try to make it look smooth – and decorate with the rainbow smarties to create a rainbow pattern. If you have some buttercream left, you can also pipe little swirls on top of the cake as decoration. Store in the refrigerator until ready to serve.