Put the berries in a bowl, sprinkle over the sugar and pour over the prosecco. Stir to combine. Leave to hang out overnight in the fridge.
Bake your cupcakes, then leave to cool. Cut the top out of the cupcakes and spoon in a bit of the macerated berries. Set aside.
To make the icing, beat the butter until pale and creamy. Add the icing sugar a half cup at a time, incorporating each bit before adding the next measure.
Simmer the berries with the cornflour over medium heat until broken down, rich in colour, thick and jam-like. Cool completely. Strain this into your buttercream and, without mixing, spoon into a piping bag. Pipe on top of your berry filled cupcakes- hiding the berries.