Preheat oven to 350 degrees F. Grease the bottom and 1/2 inch up the sides of a 9x5x3-inch loaf pan; set aside. In a large bowl, combine flour, baking powder, lemon peel (finely shredded), baking soda, salt, nutmeg, and cinnamon. Make a well in center of flour mixture; set aside.
In a medium bowl, combine eggs, mashed bananas ( 5 medium ripe bananas), sugar, melted butter, and rum (in place of rum use can use milk). Add egg mixture all at once to flour mixture. Stir just until moistened (batter should be lumpy). Transfer one-third of the batter (about 1-1/4 cups) to a medium bowl; fold in melted chocolate.
Drop alternating spoonfuls of plain and chocolate batters into the prepared pan. Using a table knife or narrow metal spatula, gently cut through batters to marble.
Bake for 55 to 60 minutes or until a toothpick inserted near the center comes out clean. If necessary, cover loosely with foil during the last 15 minutes to prevent overbrowning.
Cool in pan on a wire rack for 10 minutes. Remove bread from pan. Cool completely on wire rack. Wrap and store overnight before slicing. If desired, serve with chocolate-hazelnut spread. Makes 16 servings.