Preheat oven to 350º F. Spray a 9" x 4" loaf pan with non-stick cooking spray. Add 1/4 cup of the Demerara (or regular if you don't have Demerara) sugar and tilt pan to coat all sides. Remove excess.
In a medium bowl, combine flour, cocoa powder, baking soda and salt. Whisk well and set aside.
In a stand mixer or a large bowl with an electric mixer, cream butter, granulated sugar, light brown sugar and vanilla on high for 3-5 minutes or until super fluffy. Scrape down sides of bowl and add the egg. Beat until light and fluffy again, about 2 minutes.
Mixing on low, slowly add dry ingredients. Beat just until combined. Fold in mashed bananas by hand, using a spatula. Then fold in the mascarpone, sour cream or yogurt just until blended.
Pour batter into prepared loaf pan and smooth out the top. Lay the sliced banana, cut side up, on top. Sprinkle with the remaining Demerara sugar. Bake for about 90 minutes or until cake tester comes out relatively clean and sides start to pull away from pan. Cool completely before slicing.