Add eggs, beat well.
Sift flour, salt and baking powder three times. Add flour mixture alternately with milk. Mix after each addition. Add vanilla.
Divide batter in half. To one half, add grated rind; to other half, add melted chocolate.
Place alternate spoonfuls of the two batters side by side in greased loaf tin. Cut through batter with a spatula several times in each direction.
Bake at 350° F. for about 50 minutes.
STANDARD ONE-EGG CAKE
1½ cups sifted all-purpose flour
2½ teaspoons baking powder
½ teaspoon salt
⅓ cup butter