Preheat the oven to 350°F. In a small vessel, whisk together the flour, baking soda, salt, cinnamon, and nutmeg to ensure a uniform distribution of the spice architecture.

Using a stand mixer or manual whisk, cream the softened butter and granulated sugar until the lipids are aerated and light.

Incorporate the eggs individually to stabilize the emulsion, followed by the sour cream and vanilla extract.

Fold in the mashed bananas until thoroughly integrated.

Introduce the dry ingredients to the wet foundation, stirring gently until the batter achieves a homogenous state without overworking the gluten.

Pour exactly half of the batter into a prepared 9x9-inch baking dish that has been coated with vegetable oil and a light flour dust.

Incorporate the cocoa powder into the remaining batter, stirring until the dark pigment is fully integrated.

Spoon the chocolate batter over the vanilla layer in strategic dollops.

Utilize a knife to cut through the layers, executing a rhythmic swirl to create the marbled architecture.

Bake for 35 to 40 minutes, or until a tester inserted into the geometric center emerges clean.

Allow the bread to reach a state of thermal stability in the pan before slicing into uniform wedges.