Preheat oven to 180°C (160°C fan forced). Place butter and chocolate in a heatproof bowl over a saucepan of simmering water and stir until melted. Remove from heat

Quickly stir in egg, sugar, flour, baking powder, cocoa powder and a pinch of salt until just combined

Drop 2 teaspoons of mixture onto trays. Bake for 12-15 minutes or until just set. Cool on trays

Beat peanut butter frosting ingredients together in a bowl. Pipe onto biscuits and sandwich together