Beat the butter, sugar and vanilla with an electic beater until pale and creamy. Add the eggs, one at a time, beating well after each addition until just combined.
Fold in the flour and milk gently until mixture is smooth. Spoon the mixture into 12 x 1/3 cup paper lined muffin pans.
Bake in a moderate oven 180c for 15 minutes until cooked and golden. Allow to cool on a wire rack.
To make frosting combine the cream cheese, peanut butter and sugar and stir until combined. Gently fold through the melted chocolate until combined. Ice cooled cupcakes.