Preheat oven to 180C / 350F. Line a muffin tray with paper liners.
Stir the lemon juice into the milk to sour it and set aside.
In a large bowl, whisk the melted butter and sugar until well combined. Stir in the milk mixture and the vanilla.
Sift in the flour, baking powder and cocoa and mix to combine.
Spoon the mixture into the prepared muffin tray to ¾ full. Bake for 15-20 minutes until risen and an inserted skewer comes out clean. Allow to cool in the tray for 5 minutes, then transfer to a wire rack to cool.
Meanwhile, for the icing, beat the butter, icing sugar, vanilla and and 1 tablespoon of milk until thick and smooth. If the mixture is too thick, add another tablespoon of milk. If it’s too thin, add more icing sugar.
Spoon the icing into a piping bag and decorate the completely cooled cupcakes as desired.
The nutritional information provided is approximate and is created with online calculators. Info will vary based on cooking methods and brands of ingredients used.