Banana Loaf Cake with Cocoa Swirl Ribbon

Steps

1. Preheat the oven to 350°F (175°C). Brush a 9x5-inch loaf pan with about 1 teaspoon of the melted butter, keep the rest of the melted butter for the batter, and line the pan with a parchment sling.
2. Make the dry mix: in a medium bowl, whisk together the flour, baking soda, baking powder and salt.
3. Make the wet mix: in a large bowl, whisk the mashed banana, brown sugar, the remaining melted butter, beaten egg, sour cream and vanilla until smooth.
4. Fold the dry mix into the wet mix with a spatula just until no dry flour remains; this is the banana batter.
5. Make the cocoa filling: in a small bowl, stir the unsweetened cocoa powder, granulated sugar, cinnamon and whole milk into a thick, glossy paste, then fold in the chocolate chips.
6. Scoop about one third of the banana batter (roughly 1 cup) into the bowl with the cocoa filling and stir until evenly dark; this is the cocoa batter. The other two thirds stay plain.
7. Spread half of the plain banana batter in the pan, spoon all of the cocoa batter over it, then top with the remaining half of the plain banana batter.
8. Run a butter knife through the layers in a few S-shaped passes to swirl the cocoa ribbon, without overmixing.
9. Bake 55 to 65 minutes, tenting loosely with foil after 40 minutes if the top darkens quickly, until a toothpick in the center comes out clean or with only melted chocolate and no wet batter.
10. Cool in the pan on a rack for 15 minutes, lift the cake out by the parchment, and cool completely on the rack, about 1 hour, before cutting into 12 slices.