Preheat the oven to 180°C/350°F/Gas Mark 4. Grease the muffin moulds, dust with flour and remove excess flour.
Put the chocolate in a microwave-safe bowl, add the butter and melt in the microwave oven on HIGH for one minute. Remove the bowl from oven and stir the contents briskly. Break the eggs into another bowl. Add powdered sugar and blend with a hand blender till light and frothy. Sift the cocoa powder directly in the mixture and mix.
Add the melted chocolate-butter mixture and mix. Sift the flour and baking powder directly into the bowl and blend with the hand blender till smooth. Pour the batter into the muffin moulds, taking care not to fill up to the top.
Place the moulds in the preheated oven and bake for seven to eight minutes. Take the moulds out of the oven, loosen the edges carefully with a knife. Carefully unmould the cakes onto a serving plate and serve hot.