Place 16 mini silicone muffin cases on a baking sheet.
Put the butter, sugar, flour, vanilla extract and egg in a bowl and beat with a hand-held electric whisk until light and creamy. Divide among the cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes until risen and just firm. Leave to cool in the cases for 2 minutes then transfer to a wire rack to cool completely.
Make the buttercream by beating together the butter and icing sugar until combined. Add a little pink food colouring and the measurement water and beat until smooth and creamy.
Use a small, sharp knife to cut out circles from the tops of the cakes and cut the circles in half to shape butterfly wings. Put the buttercream in a piping bag fitted with a small star nozzle and use to pipe swirls into the scooped-out tops of the cakes. Position the butterfly wings on top and dust lightly with icing sugar.