In a separate bowl, beat eggs until frothy, and gently add to butter and sugar.
Add in flour, baking powder and lemon juice.
Pour 2/3 of the batter into a greased and floured cake pan (I'm assuming a 9 inch spring form or 13x9 pan would work.)
Melt the chocolate, and mix into the remaining batter. (You may need a tablespoon of cream to thin the chocolate as you melt it.) Pour over the batter in the pan, and make a marbled pattern using a fork.