Preheat oven to 350°F (180°C). Spray an 8½x4½-inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over sides of pan.
In a medium bowl, whisk together flour, salt, baking soda, and baking powder.
In a large bowl, whisk together banana, brown sugar, oil, sour cream, eggs, melted butter, and vanilla. Gently stir in flour mixture just until combined and no dry spots remain.
Transfer one-third of batter (about 1 cup or 281 grams) to a small bowl; stir in ¼ cup (66 grams) peanut chocolate spread. Using a 1½-tablespoon spring-loaded scoop, scoop batters into prepared pan in alternating pattern. Spoon remaining 2 tablespoons (33 grams) spread on top of batter; using a skewer or wooden pick, gently swirl spread into batter.
Bake until a wooden pick inserted in center comes out clean, 1 hour to 1 hour and 10 minutes, loosely covering with foil during final 15 minutes of baking to prevent excess browning. Let cool in pan on a wire rack for 10 minutes. Using excess parchment as handles, remove from pan, and let cool completely on a wire rack. Store in an airtight container for up to 3 days.