1. Preheat oven to 325 degrees Fahrenheit . Grease & flour an 8 1/2 x 4 1/2 loaf pan.
2. Melt chocolate & 2 Tablespoons of butter in medium size microwave safe bowl. Set aside.
3. Mash bananas in small bowl. Add lemon juice & zest. Set aside.
4. Whisk flour, baking powder, salt,& nutmeg in small bowl. Set aside.
5. Beat butter & sugars together on medium speed until creamy; About 3 minutes. Add eggs one at a time, Beating well after each addition. Beat in vanilla.
6. Reduce speed to low and add half of dry mixture. Beat until just incorporated. Mix in milk until just blended. Add rest of dry mixture.
7. Scrape down the bowl &add in banana mixture.
8. Pour a little less than half of batter into bowl with the melted chocolate and stir to blend.
9. Drop alternating spoonfuls of both batters into the prepared pan. (Or you could use other marbling techniques) Using an table knife zig-zag through the batter 6-8 times; taking care not to over-do it.
10. Bake for 1 hour & 20 to 30 minutes until a thin knife inserted comes out clean. (If cake starts to brown too fast, cover loosely with tented foil.).
11. Transfer pan to cooling rack & let it rest for 15 minutes before unmolding. Cool cake to room temperature before serving,.