Method

Preheat the oven to 180 ºC and line a 28 x 18 cm cake tin with baking paper.

Sift the flour, sugar, baking powder, bicarb and salt together. Add to the bowl of a stand mixer fitted with a paddle attachment and mix on low for a few turns to disperse the ingredients evenly. Add the butter and mix on low to incorporate. This method is called reverse creaming, the butter fat is coated by the flour before the addition of the eggs. This creates a barrier that slows the formation of gluten and tt results in a tender texture and a flatter top. 

Mix the buttermilk, eggs and vanilla essence together in a bowl. Add this to the dry ingredients in three batches, mixing on low and ensuring that you scrape down the sides in-between. Don’t overmix but ensure that you have a smooth batter.

Remove the bowl from the stand mixer and use a rubber spatula to fold in the rainbow sprinkles. Pour the batter into the prepared tin and place into the oven to bake for 25 mins or until the top is lightly golden and a cake tester or toothpick inserted into the centre comes out clean. 

Remove from the oven and allow to cool completely. Lift or turn out of the tin and remove the baking paper. Once cooled, trim the sides to reveal the pretty sprinkles.

Icing

Add the margarine to the bowl of a stand mixer, fitted with the paddle attachment. Beat the margarine until smooth. Add the icing sugar in 3 batches and mix to incorporate. Add the vanilla and milk and mix to combine. The icing should be smooth and fluffy. 

To Assemble

Trim the edges of the cake just enough to reveal the pretty rainbow inside. Ice the top of the cake – you can leave the sides naked or choose to ice them as well. Use a piping bag and nozzle to decorate the top of the cake and decorate with additional confetti sprinkles.