Beat butter with sugar until light and fluffy. Sift together the baking powder and the flour. Add 1 tablespoon of the flour to the the butter mixture and mix. Gradually add eggs, beating all the time. Fold in remaining flour.
Transfer half of mixture to another bowl and mix cocoa into this. Stir in the grated lemon rind.
Spoon half the plain mixture into a 4 cup greased, plastic ring mold, then cover with half the chocolate mixture. Repeat layers, finishing with the chocolate mixture.
Microwave on HIGH (100%) for 4 minutes or until the cake is just dry on top. Let cake stand for 3 to 4 minutes, before serving.