Heat oven to 190C/fan 170C/gas 5 and put 16 gold cases into a fairy-cake tin. Tip all the ingredients for the cake into a mixing bowl and beat for 2 mins with an electric hand-whisk until smooth. Divide between the cases so they are two-thirds filled, then bake for 12-15 mins until risen. Cool on a wire rack.
For the frosting, microwave the chocolate on High for 1 min. Cream the butter and sugar together, then beat in the melted chocolate. Spread on the cakes and decorate with Maltesers and chocolate eggs.