Preheat oven to 350°. Lightly coat a loaf pan with melted butter. Then line with parchment paper, making sure to leave a generous overhang.
In a medium bowl whisk flour, baking soda, and salt. Set aside.
In a bowl of a stand mixer, beat brown sugar, mascarpone, and butter on medium high until very light and fluffy, about 4 minutes.
With the mixer on low, add the eggs one at a time, beating to blend completely after each addition. Scrape down sides and bottom of bowl as needed with a spatula.
Keeping the mixer on low, add the flour mixture all at once. Mix until just combined.
Add the bananas and mix just until combined.
With a spatula, fold in chocolate and/or walnuts or pecans. Pour batter into prepared loaf pan, using the spatula to smooth the top. Sprinkle with Demerara sugar.
Bake bread until a toothpick inserted into the center comes out clean, about 60–65 minutes. Transfer the pan to a wire rack and let it cool in pan for 1 hour.
Turn out bread onto a wire rack and let cool completely before slicing. This will be hard!! I made it a whole 10 minutes before I sliced into it.