Preheat your oven to 180℃ (160℃ for fan ovens) and line a cupcake tin with 12 cases.
In a large bowl, cream together the butter and sugar until soft and fluffy. Stir in the vanilla extract.
Mix in the flour and the eggs, a little bit at a time, until you have a smooth, thick cake batter. Then stir in the lemon zest. If needed, you can add a splash of milk to help loosen the mixture.
Divide the cake mix between your 12 cases and bake for 20-25 minutes until a toothpick comes out clean. Transfer them to a wire rack and leave to cool completely.
Once the cupcakes are cool, gently cut the tops off using a small serrated knife. Cut the tops in half then set them aside with their matching cupcake.
Topping:
Add the softened butter to a bowl and slowly stir in the icing sugar, a little at a time. Add the lemon juice and lemon zest and mix until smooth.
Transfer the buttercream to a piping bag fitted with a star nozzle and pipe a swirl onto the flat top of each cupcake. Then stick on the two halves of the tops with the flat side down, to create a butterfly shape.