Preheat oven:
Set to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper.

Mix dry ingredients:
In a bowl, whisk together flour, baking soda, and salt.

Mix wet ingredients:
In a large mixing bowl, combine mashed bananas, sugars, melted butter, egg, and vanilla until smooth.

Combine wet & dry:
Stir the dry ingredients into the wet mixture until just combined (don’t overmix).

Make red velvet swirl batter:
Scoop out about 1 cup of the banana batter into a separate bowl.
Add cocoa powder, buttermilk, and red food coloring. Mix until smooth.

Layer and swirl:
Pour half of the plain banana batter into the loaf pan.
Spoon some of the red velvet batter on top.
Alternate layers, then use a knife to gently swirl the two batters together.

(Optional: add dollops of cream cheese mixture for a cheesecake-like swirl.)

Bake:
Bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean.

Cool:
Let it cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.