Preheat oven to 180C/350F/Gas 4. Line cupcake pan with 20-22 cupcake liners. Set aside.
Combine milk and lemon juice to create buttermilk.
Sift together the flour, sugar, baking powder, and salt.
Add the butter, vanilla extract, and buttermilk to the flour mixture. Beat to combine.
Add egg whites to the mixture and beat for 2 minutes.
Scoop into cupcake liners to 3/4 full.
Bake for 20-22 minutes or until an inserted toothpick comes out clean.
Leave the cupcakes in the pan to cool for 5 minutes before transferring to a wire rack to cool completely before decorating.
To make the buttercream frosting, blend all the ingredients in a bowl using an electric stand or hand-held mixer.
Fill a piping bag with star tip and pipe on top of cupcakes as desired.
Add sprinkles and unicorn decorations and enjoy!