Preheat oven to 325°F (170°C). Spray an 8 1⁄2×4 1⁄2-inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over all sides of pan.
For batter: In a medium bowl, whisk together flour, salt, baking powder, and baking soda.
In a large bowl, whisk together banana, brown sugar, peanut butter, sour cream, oil, eggs, and vanilla. Gradually stir in flour mixture just until combined. Spread batter into prepared pan.
Bake for 50 minutes. Rotate pan, and bake until a wooden pick inserted in center comes out clean and an instant- read thermometer inserted in center registers 200°F (93°C) to 205°F (96°C), 35 to 45 minutes more, loosely covering with foil during final 20 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 wire rack. Wrap cooled loaf in plastic wrap, and refrigerate for 1 hour.
Unwrap loaf, and place on wire rack on a rimmed baking sheet.
For topping: In a medium microwave- safe bowl, heat all ingredients on high in 30-second intervals, stirring between each, until chocolate and butter are melted and mixture is well combined. Pour topping all over top and sides of loaf, spreading with an offset spatula to cover. Refrigerate until set, about 10 minutes, before serving. Store in airtight container for up to 3 days.
Source: https://www.bakefromscratch.com/category/recipes/bread/feed/
