In a large mixing bowl, beat together the butter and sugar until very light and fluffy. Gradually add the beaten eggs, beating well after each addition. In a measuring jug or bowl, stir together the vanilla extract, milk and sour cream. Sift half of the flour over the creamed butter mixture and lightly fold in using a large metal spoon. Fold in half the sour cream mixture, taking care not to over mix. Repeat with the remaining flour and sour cream mixture, again being careful not to over mix. Spoon the batter into your cupcake liners (no more than two thirds full) and bake for 20-25 minutes or until light golden and spring back when lightly pressed. Leave in the tin for 5 minutes, then turn out to cool completely on a wire rack.

Using a medium bowl set over a pan of simmering water, (making sure the base of the bowl does not touch the water) melt the white chocolate. As soon as the chocolate has melted, remove from the heat and stir until smooth. Set aside to cool a little.

In a large bowl (preferably with an electric mixer) beat the butter until creamy. Gradually add the icing sugar, about 100g at a time. When the icing sugar is all incorporated, beat on high speed until very light and smooth. Lower the speed and add the cooled melted white chocolate, vanilla extract and 2 tablespoons of double (heavy) cream and beat on medium until nice and smooth.