Preheat the oven to 180ºC/Fan 160ºC/Gas 4. Line a 12-hole cupcake tray with paper cases. (White or a mixture of pink and yellow looks good).
Put the butter, sugar, vanilla, egg, flour and baking powder in a bowl and whisk with an electric mixer until smooth and creamy. Add the milk and mix again until combined.
Spoon heaped teaspoonfuls into the cases and bake for 18-20 minutes until risen, pale golden in colour and just firm to the touch. Transfer to a wire rack to cool.
For the buttercream, beat the butter, icing sugar and vanilla in a bowl until very pale. Add 1 tsp hot water and beat again until very soft. Spoon half the mixture into a separate bowl. Beat a drop of red food colouring in one bowl and yellow into the other. Spread or pipe the buttercream over the cooled cakes. Place a few sweets on top of each cupcake.