1
Preheat oven to 350ºF. Spray an 8.5 x 4.5 inch loaf pan with nonstick cooking spray.
2
Add the butter, granulated sugar, and brown sugar to the bowl of an electric mixer; mix on medium speed until light and fluffy, about 3 minutes. Add the eggs, bananas, and yogurt; beat 1 minute. Add the flour, baking soda, salt and vanilla, beat until airy and smooth, about 1 minute more.
3
Remove 3/4 cup of batter to a small bowl. Add the melted choclate chips; mix until smooth.
4
Spoon the banana batter into the prepared loaf pan, then spoon the chocolate batter on top; use a table knife to swirl the two together.
5
Bake for 1 hour, or until a toothpick inserted in the center comes out clean. Cool 10 minutes. Remove from the pan to cool thoroughly before cutting.