Preheat oven to 325 degrees F. Grease and flour one 9×5 inch loaf pan.
In a medium bowl, whisk together flour, baking powder, salt, and nutmeg; Set aside.
In small bowl, mash the bananas with the lemon juice and zest. Stir in rum.
In a small, microwave-safe bowl, melt chocolate and 2 tablespoons of butter in the microwave on high power for 45 seconds. Stir. If un-melted pieces remain continue microwaving and stirring the chocolate in ten-second intervals until it is all melted and smooth; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter until creamy, about 3 minutes. Add sugars and beat until light and smooth, about 2-3 minutes. Add eggs one at a time, beating well after each addition. Add vanilla and beat until combined. On low speed, add half of the flour mixture and beat until incorporated. With the mixer still running, pour in milk and mix until blended. Add remaining flour and banana mixture and mix on low until just combined.
Pour half of the batter into the bowl with melted chocolate and stir to combine. Drop alternating spoonfuls of both batters into the prepared pan, then, using a table knife, swirl the batters together, taking care not to overdo it.
Bake for 1 hour and 20 to 30 minutes, or until a toothpick inserted in the center of the loaf comes out clean. Cool in pan 15 minutes, then transfer to wire rack and cool completely before serving, at least 1 hour.