Line 4 cupcake molds with paper or foil cupcake liners; set aside. Stir the chocolate and butter in a heavy small saucepan over medium-low heat until melted and smooth, about 1 minute. Remove from the heat.
Using an electric mixer, beat the sugar, egg and egg yolk in a large bowl until thick ribbons form on the surface of the batter when the beater is lifted, about 5 minutes. Sift the flour over the egg mixture then fold in the flour. Fold in the melted chocolate mixture.