1 1/3 cups all-purpose flour, 1 tsp baking powder, 1/2 tsp salt, 1/4 tsp freshly grated or ground nutmeg, 1 1/2 ripe medium bananas, 1/2 tsp finely grated lemon zest, small squirt of lemon juice, 1 Tbsp dark rum, 3 oz bittersweet chocolate, finely chopped, 1 stick plus 2 Tbsp (1/2 cup plus 2 Tbsp) unsalted butter, softened, 2/3 cup packed light-brown sugar, 1/3 cup granulated sugar, 4 large eggs, 1 tsp vanilla extract, 1/2 cup whole milk
Heat oven to 325°F. Butter an 8×4 x 21/2-in. loaf pan; dust with flour; tap out excess. Whisk flour, baking powder, salt and nutmeg in a small bowl until blended.
In another small bowl, mash bananas with lemon zest and juice; stir in rum. Microwave chocolate and 2 Tbsp butter, stirring every 10 seconds, until melted and blended.
Beat remaining butter with mixer on medium speed 3 minutes or until creamy. Add sugars; beat 2 to 3 minutes until light and smooth. Add eggs 1 at a time, beating well after each. Beat in vanilla (batter will look curdled). Reduce mixer speed to low and beat in 1/2 the flour mixture just until blended. Beat in milk until combined, then remaining flour mixture. Scrape down bowl; beat in banana mixture (batter will look lumpy).
Stir a little less than 1/2 the batter into melted chocolate to blend. Drop alternating spoonfuls of both batters into prepared pan. Using a table knife, swirl batters together, taking care not to overdo it.
Bake 1 hour and 20 to 30 minutes (check cake after 45 minutes and cover loosely with foil if it’s getting too brown), until a wooden pick inserted in center comes out with moist crumbs attached. Cool in pan on a wire rack 15 minutes before inverting on rack, turning right side up and cooling completely.