Preheat oven to 350°F. Grease a 9"x5" loaf pan and set aside.
In a large bowl, combine flour (s), baking powder, lemon zest, salt, nutmeg and cinnamon. Make a well in the center of the flour mixture, set aside.
In a medium bowl, combine eggs, mashed bananas, sugar, melted butter and milk.
Add the wet ingredients to dry ingredients and stir just until moistened.
Pour 1/3 of the mixture to a small bowl and fold in the melted chocolate.
Drop alternating spoonfuls of plain and chocolate batters into the prepared pan. Using a table knife or a metal spatula, swirl around the batters to marble.
Bake for 50-60 minutes or until a toothpick inserted in the center comes out clean.If the top seems to be browning too quickly, then cover the pan loosely with a foil tent.
Cool the bread in the pan for 10 minutes, then remove it onto a wire rack.