Method

1. Preheat oven to 175°C and line a muffin pan with cupcake liners, you will need 2 muffin pans for 14 cupcakes. Set aside.
2. To make the cupcakes whisk the flour, baking powder, bicarb soda and salt together. Set aside.
3. With an electric mixer beat the butter on high speed until smooth and creamy, approximately 1 minute. Add the sugar and beat on high speed for another 2 minutes, until creamed together. Scrape down the sides of the bowl as required. Add the egg whites and beat on medium high until just combined, then beat in the sour cream. With the mixer on low speed add the dry ingredients until just combined. With the mixer still on low slowly add the milk until combined. Do not overmix.
4. Pour or spoon batter into the cupcake liners, filling until 2/3 full.
5. Bake in the oven for 19 – 22 minutes. Allow the cupcakes to cool in the pan before removing to a cooling rack.
6. To fill the cupcakes using a sharp knife, cut a hole into the centre of the cupcake ensuring you do not cut all the way through. Remove the piece of cake and set aside. Place about 1 teaspoon of sprinkles inside the cupcake. Slice of the pointy end of the removed cupcake so that you can fit the piece of cupcake on top of the sprinkle filling. Repeat with each cupcake.
7. To make the frosting use an electric mixer to beat the butter on medium speed for about 2 minutes, until creamy. Add 4 and ½ cups of icing sugar, the milk and vanilla extract mixing on low and increasing to high speed for 3 minutes. Add an extra ½ cup of icing sugar if frosting is too thin or add more milk if the frosting is too thick. Add a pinch of salt if the frosting is too sweet. Frost the cupcakes with your preferred pattern and serve.