Preheat oven 175°C fan forced. Grease and line 2 x 20cm cake tins.
In a large bowl, sift together the cake flour, baking powder, bicarb soda and salt. Mix well and set aside.
In a stand mixer with a paddle attachment, beat the butter on high for 1 minute. Add the sugar and continue to beat on high speed for 5 minutes, scraping down the sides of the bowl as required.
On medium speed, add whole eggs one at a time, beating well between each one. Add the vanilla extract.
On low speed, add a 1/3 of the dry ingredients, alternating with 1/3 of the buttermilk and mix until just combined. Do not overmix the batter.
In a separate bowl, whisk the 2 egg whites until soft peaks form, approx. 3 minutes and then gently fold into the cake batter. Add the sprinkles and gently fold these through the batter.
Spoon the batter evenly into 2 x round pans (approx. 900g into each tin) and bake for 40-45 minutes. Test with a wooden skewer to ensure it comes out clean.
Allow cakes to cool completely in the pans set on a wire rack before frosting.
To make Buttercream icing: On med–high speed, beat the unsalted butter for 8-10 minutes until pale and fluffy. Add 2 cups of the icing sugar and mix for 2 minutes, then add another 2 cups of icing sugar and mix for another 2 minutes. Add the vanilla, 3 tsp milk and the remaining 3 cups of icing sugar to the mixer. Continue to mix for another 2 minutes until icing sugar is fully incorporated. Add milk if required to the buttercream for a more spreadable consistency.
To assemble and frost: Using a large, serrated knife or a wire cake cutter, cut each cake through the middle, which will create four layers of cake. If cake top is domed, trim a little off the top to create a flat surface.rnrnPlace 1 cake layer on a cake stand and evenly cover the top with buttercream. Repeat with the remaining layers of cake. Using a piping bag and a Wilton 1M piping tip, pipe the buttercream around the top edge of the cake and sprinkle with funfetti.rn