Line a 12-hole muffin tin with paper cases and heat oven to 190C/fan 170C/gas 5. In a jug, mix the yogurt, eggs and vanilla extract. Put the dry ingredients, plus a pinch of salt, into a large bowl and make a well in the middle.
Add the yogurty mix and melted butter, and quickly fold in with a spatula or metal spoon – don’t overwork it. Spoon into the
cases (they will be quite full) and bake for 18-20 mins or until golden, risen and springy to the touch. Cool for a few mins, then lift the cakes onto a wire rack to cool completely. Keep in an airtight container for up to 3 days or freeze as soon as possible.
Chocolate frosting: Melt the chocolate in the microwave on High for 1½ mins, stirring halfway. Leave to cool. Beat the butter and icing sugar in a large bowl until creamy. Beat in the chocolate. Cover and chill for up to one month. Up to 48 hrs before serving (or the day before if it’s really hot), bring back to room temperature, then spread over the cakes. Put the chocolate buttons on. Keep cool, out of direct sunlight.