Using an electric mixer, beat butter and sugar until light and fluffy, add eggs one at a time, beating well after each, then add white chocolate and stir to combine. Sift flour and baking powder over cake batter and fold in until combined. Line two 6-hole (1/3 cup-capacity) muffin tins with 8 paper cases, then divide mixture among cases and bake at 180C for 20 minutes or until cakes spring back when lightly pressed.
For raspberry frosting, using an electric mixer, beat butter until pale and fluffy, add icing sugar and beat until soft peaks form, then add raspberry purée and beat until combined. Spread frosting over cupcakes and serve. Cupcakes will keep in an airtight container for up to 3 days.