Pre-heat oven to 350f/180 c. Grease loaf pan with oil or butter.
In a small bowl, mix whole milk and vinegar or lemon. Stir then set aside until it curdles.
In a large mixing bowl, combine flour, salt, baking powder, and baking soda together.
In another bowl, cream butter and sugar. Whisk until mixture becomes pale and smooth. You can do this by hand or use an electric mixer. Stir in eggs one at a time.
Add buttermilk, vanilla and mashed bananas to the butter mixture. Stir until just combined.
Combine wet mixture to the dry mixture. Stir until well blended. Divide batter between two bowls.
Melt chocolate in the microwave for 30 seconds in two intervals. You can also melt it in a small pot over low heat. Stir in the cocoa powder. Add chocolate mixture to one portion of the batter, mix well.
In the loaf pan, alternate dollops of the plain batter and chocolate batter. Using a skewer, swirl the batter a few times to create a marbled effect. (see video)
Bake for 45 minutes to 50 minutes until a skewer inserted into the center comes out clean. Mine took exactly 50 minutes to get done. Let it cool for 15 minutes before removing from the pan. Slice and serve. Enjoy!
Video
Notes
Cool completely before slicing to prevent breaking the cake.
Shelf life and storage: This will keep for 5 days in the fridge in an airtight container. Wrap in plastic or foil to seal in moisture.
Don’t use brown sugar to keep the light color of the yellow batter.
Don't use overripe bananas as they tend to darken after baking the cake.
If you're watching your sugar intake, reduce the sugar up to 1 cup. It will still be delicious. I promise!