Step 1

Preheat the oven to 350°F. Grease a 9 x 5-inch loaf pan generously with butter or nonstick cooking spray.

Step 2

In a medium bowl, whisk together the flour, baking soda and salt. Set aside.

Step 3

In the bowl of an electric mixer, beat the butter and sugar until light and fluffy, about 2 minutes. Add the eggs, one at a time, beating well after each addition. Add the flour mixture and mix on low speed until just combined. Add the mashed bananas, sour cream and vanilla extract and beat until just combined. Do not overmix.

Step 4

Place the chocolate in a microwave safe bowl. Cook on high power in 20 second intervals, stopping to stir in between, until about 80% melted. Stir until smooth, letting the residual heat in the bowl melt the remaining bits of unmelted chocolate. Add the cocoa powder and stir until dissolved. Add one cup of the banana bread batter to the melted chocolate and stir until well combined.

Step 5

Spoon the plain batter and chocolate batter alternately into the prepared loaf pan. Swirl the batters together with a knife (don't overdo it or the colors will get muddled -- up and back 4 to 5 times should suffice). Bake for about 1 hour and 10 minutes, or until a cake tester comes out clean. Cool in the pan on a wire rack for about 10 minutes, then remove the loaf from the pan and set on the wire rack to cool completely.

Step 6

Freezer-Friendly Instructions: The bread can be frozen for up to 3 months. After it is completely cooled, wrap it securely in aluminum foil, freezer wrap or place in a freezer bag. Thaw overnight in the refrigerator before serving.