Preheat the oven to 180°C, Fan oven 160°C, Gas Mark 4. Put 10 large paper cupcake cases into a bun tin.
Beat the butter and sugar together until light and creamy, then gradually beat in the eggs. Stir in the milk and vanilla extract, then sift in the flour, cocoa, baking powder and salt, folding in with a large metal spoon.
Spoon the mixture into the paper cases. Bake on the middle shelf of the oven for 22-25 minutes, until risen and firm. Cool for a few minutes, then transfer to a wire rack to cool completely.
To decorate, beat the butter, icing sugar and cocoa powder together for 3-4 minutes until light and fluffy, adding 1 tbsp warm water. Pipe or spread on top of the cakes, leaving a slight depression in the middle. Sprinkle chocolate flake around the edges, then place 3 mini chocolate eggs in the middle of each cake.