Preheat the oven to 325 degrees F. Make sure the rack in the center.

Grease and flour an 8 ½ x 4 ½ x 2 ½ inch loaf pan. Line with a strip of parchment if desired, then grease the parchment and dust with more flour. Set the loaf pan on two stacked baking sheets (or use one insulated sheet). This is supposed to protect the bottom from burning.

Whisk together the flour, baking powder, salt and nutmeg.

In a small bowl, mash the bananas with the lemon juice and zest (if using), then stir in the rum if using.

Melt the chocolate and 2 tablespoons of the butter together in the microwave or using your favorite method.

With an electric mixer (I use a handheld), beat the remaining stick (8 tablespoons) of butter at medium speed until creamy. Add the sugars and beat for another 2 to 3 minutes, until light and smooth. Add the eggs one at a time, beating well after each addition, then beat in the vanilla. The batter will look curdled, and it will continue to look curdled as you add ingredients. By hand or using the lowest speed of mixer, add half the flour mixture. Pour in the milk and stir until blended, then add the remaining flour mixture. Scrape down the bowl and mix in the mashed bananas.

Pour a little less than half the batter (I weighed out 16 oz) and blend with the melted chocolate to make a chocolate batter. Drop alternating spoonfuls of both batters into the prepared pan, then very carefully draw a knife through the batter in a zig-zag fashion just to get a marble effect going.

Bake for 80 to 90 minutes, or until a knife inserted deep into the center of the cake comes out clean. NOTE: Check after 30 minutes and if the cake starts to brown too much, cover it loosely with a foil tent.

Transfer the cake to a cooling rack and let it rest for about 15 minutes before unmolding (I let mine cool longer), then let the cake cool to room temperature directly on the rack.