Place 16 mini silicone muffin cases on a baking sheet.
Put the butter, sugar and egg in a bowl, sift in the flour and cocoa powder and beat with a hand-held electric whisk until light and creamy. Stir in the sour cherries and divide among the cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes until risen and just firm. Leave in the cases for 2 minutes, then transfer to a wire rack to cool completely.
Make the topping by melting the chocolate (see