Preheat the oven to 180°C, 170°C fan, Mk 4. Line the mini muffin tin with papers, or prepare with grease and flour.
Put the chocolate and butter in a non metallic heatproof bowl. Set over a pan of simmering water, and melt the chocolate and butter together. Allow to cool slightly.
Whisk the egg with the Billington’s dark muscavado sugar well, until blended. Stir in the melted chocolate and butter. Dissolve the espresso powder in the boiling water and stir this in. Fold in the flour and baking powder until the mix is even. Spoon into the mini muffin tin and bake for 10 – 12 minutes.
Meanwhile, mix together the mascarpone and icing sugar. To serve, spoon a little of the mascarpone frosting on top of each mini muffin and sift over the cocoa. Serve with coffee.