Preheat oven to 350. Spray a 9x5" loaf pan with nonstick spray. Set aside.
In a medium bowl, whisk together baking blend, cornstarch, baking soda, and baking powder. Set aside. In a large bowl or stand mixer, beat together mashed bananas, applesauce, sugars and vanilla until well combined. Add half the dry ingredients, followed by buttermilk, then remaining dry ingredients. Mix until just combined. Fold in white chocolate chips
Pour half the batter into prepared loaf pan. Pour 1 cup melted peanut butter on top in an even layer. Top with remaining batter. Add a drizzle of extra melted peanut butter if desired.
Bake until golden and set, and a toothpick inserted in the center comes out mostly clean, about an hour. Cool completely. Store leftovers in an airtight container for up to 1 week.