1. Preheat the oven to 180°C (350°F). Line two 20cm springform tins* with baking parchment and grease lightly with vegan butter or oil. Set aside. Mix the oat milk with the apple cider vinegar and set aside for 5-8 minutes so it can curdle a bit. Measure a cup (120ml) of the liquid (aquafaba) from a can of chickpeas and add it to a bowl. Mix with a handheld mixer for 2 minutes until you get a thick foam almost like whisked egg whites. Set aside until needed.

2. Add the soft vegan butter and sugar to a large bowl and mix on high speed until light and fluffy. Add the oil and vanilla extract and mix until well combined. Add the aquafaba to the bowl and mix on high speed until everything is light and fluffy and well combined. Mix the flour with baking powder and salt. Add in three batches to the bowl, alternating with the curdled oat milk, and mix on low speed until just combined. Add the rainbow sprinkles and fold them in. Divide the batter between the two springform tins, smooth out the top and bake for 30-35 minutes or until a wooden skewer inserted into the center of a cake comes out clean. Take it out of the oven and let it cool down on a wire rack.

3. For the buttercream add the oat milk to a saucepan. Take 3-4 tbsp. of that milk and mix with the cornstarch, sugar, and vanilla extract in a small bowl until well combined. Heat up the remaining milk in the saucepan until almost cooking, then add the cornstarch mixture and stir to combine. Continue stirring until the oat milk starts to thicken and look like a pudding. Remove from the heat, place a piece of plastic wrap on the surface of the pudding and let it cool down completely. While the pudding cools down, take the vegan butter out of the fridge and let it come to room temperature. As soon as the pudding has cooled down completely, add the vegan butter and confectioners‘ sugar to a large bowl and mix for several minutes until light and fluffy. Gradually add the cooled pudding to the bowl and mix well after each addition. If you go too fast, the butter might curdle a bit – in that case, continue mixing until the cream is smooth again. When all the pudding is added, continue mixing on high speed for another 2-3 minutes until the buttercream looks very light and creamy.

4. To assemble the cake, remove the cakes from the tins, place the first layer on a serving plate, add a good amount of the buttercream on top and spread evenly on the cake. Place the second cake layer on top and crumb coat the whole cake with a thin layer of buttercream. Place in the fridge for about 20 minutes. Take it out of the fridge and coat the cake completely with the remaining buttercream. Decorate with some vegan sprinkles and place once more in the fridge for at least 30 minutes before serving.