Step #2 Butter an 8x4x2 1/2 inch loaf pan; dust with flour; tap out excess.
Step #3 Whisk flour, baking powder, salt & nutmeg in a small bowl until mixed.
Step #4 In another small bowl, mash bananas with lemon zest & juice; stir in the rum.
Step #5 Microwave chocolate & 2 T butter, stirring every 10 seconds, until melted & mixed.
Step #6 Beat remaining butter with mixer on medium speed 3 mins or until creamy.
Step #7 Add sugars; beat 2-3 mins until light & smooth.
Step #8 Add eggs, 1 at a time.
Step #9 Beat in vanilla (batter will look curdled).
Step #10 Reduce mixer speed to low & beat in 1/2 the flour mixture just until mixed.
Step #11 Beat in milk until mixd, then remaining flour mixture.
Step #12 Scrape down bowl; beat in banana mixture (batter will look lumpy).
Step #13 Stir a little less than 1/2 the batter into melted chocolate to mix.
Step #14 Drop alternating spoonfuls of both batters into prepared pan.
Step #15 Using a table knife, swirl batters together, taking care not to overdo it.
Step #16 Bake 1 hr & 20 to 30 mins (check cake after 45 mins & cover loosely with foil wrap if it's getting too brown), until a wooden pick inserted in center comes out with moist crumbs attached.
Step #17 Cool in pan on a wire rack 15 mins before inverting on rack, turning right side up & cooling completely.