1. Preheat oven to 350°. Lightly coat 8½x4½" loaf pan with nonstick spray and line with parchment paper, leaving a generous overhang on long sides. Whisk flour, baking soda, and salt in a medium bowl.
2. 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 and scraping down sides and bottom of bowl as needed.
3. Reduce speed to low, add flour mixture, and mix until just combined. Add bananas and mix just until combined. Fold in chocolate and/or walnuts, if using. Scrape batter into prepared pan; smooth top.
4. 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 1 hour. Turn out bread and let cool completely before slicing.