- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter the sides of two 8-inch (20 cm) springform pans with removable bottoms. Line the bottoms with parchment paper.
- In a bowl, combine the flour, baking powder, baking soda and salt.
- In another bowl, combine the buttermilk, vanilla and vinegar.
- In a third bowl, whisk the sugar and oil with an electric mixer for 2 minutes. Add the egg whites. With the machine running on low speed, add the dry ingredients alternating with the buttermilk mixture and whisk until smooth.
- Add ¾ cup (180 ml) sprinkles to the cake batter and mix to combine. Divide the batter between the two prepared cake pans.
- Bake for 40 to 45 minutes or until a toothpick inserted in the centre of a cake comes out clean. Let cool. Unmould and let cool completely on a wire rack, about 3 hours.
- In a bowl placed over a pot of simmering water, heat the sugar and egg whites, stirring constantly, until the sugar has dissolved. Remove the bowl from the pot. Whisk with an electric mixer until stiff peaks form. The mixture should be lukewarm. Add the vanilla.
- Add one or two pieces of butter at a time while whisking constantly. Continue to whisk for 5 minutes until the mixture is light and creamy (see note). Using a spatula, scrape down the sides and bottom of the bowl a few times (see note).
- Set aside ¾ cup (180 ml) of the vanilla icing in a pastry bag fitted with a star tip.
- Cut off the rounded top of each cake to make them as flat as possible. Cut each cake in half horizontally to have a total of 4 cake layers.
- On a serving dish, place one cake layer. Cover with one-quarter of the remaining icing. Repeat this step by stacking two more cake layers and covering each with icing. Top with the last cake layer with no icing.
- Using a spatula, apply a thin layer of icing around the sides of the cake and spread the remaining icing over the top of the cake. Using the pastry bag of reserved icing, decorate the top of the cake with rosettes of icing and sprinkle with the remaining sprinkles. The cake will keep for 3 days, covered, at room temperature.
To avoid wasting the egg yolks, use liquid egg whites. An egg white weighs 30 g and measures 2 tbsp (30 ml). Therefore, you will need ½ cup (120 g) egg whites for the cake and 1 cup (240 g) for the vanilla icing.
The mixture may separate after you add about half of the butter. Everything should come together once you add the remaining butter. This step takes time, so don’t add the butter too quickly.
If the icing seems too soft when you are ready to apply it to your cake, refrigerate it for a few minutes, then whisk it up again.
The icing can be prepared in advance if left covered at room temperature. Mix the icing well before applying it to your cake.