Preheat oven to 350°. Lightly coat an 8 1/2" x 4 1/2" loaf pan with nonstick spray and line with parchment paper, leaving about 2" of overhang on long sides. Whisk flour, baking soda, and salt in a medium bowl.
Using an electric mixer on medium-high speed, beat brown sugar, mascarpone, and butter in a large bowl until light and fluffy, about 4 minutes. Add eggs one at a time, beating to blend after each addition. Add vanilla and blend, scraping down sides and bottom of bowl as needed.
Reduce speed to low, add dry ingredients, and mix until just combined. Add mashed bananas and mix just until combined. Fold in chocolate and/or walnuts, if using. Scrape batter into prepared loaf pan; smooth top. Optional: sprinkle a dusting of turbinado sugar on top of loaf.
Bake bread until a tester inserted into the center comes out clean, 60–65 minutes. Transfer pan to a wire rack and let bread cool in pan 15 minutes. Lift bread out using parchment paper and allow to cool 5 more minutes before slicing. Serve with whipped cream cheese or butter.
