Put 8 paper baking cups in a muffin pan. Preheat oven to 375 degrees
Put the margarine, sugar, egg, flour, and cocoa in a large bowl and, using an electric beater, beat together until just smooth.
Spoon half of the batter into the paper cups. Using a teaspoon, make an indention in the center of each cake. Break the chocolate evenly into 8 squares and place a piece in each cake, then spoon the remaining batter on top.
Bake for 20 minutes, or until well risen and springy to the touch. Let stand for 2-3 minutes before serving warm. Dust with sifted confectioners' sugar