Preheat oven to 350 degrees F.
Line a 9x9” pan with parchment paper and grease with cooking spray.
In a large bowl, mix together flour, baking powder, and salt. In another large bowl, mix together melted butter, eggs, brown sugar, granulated sugar, and ¾ c. peanut butter. Gradually fold dry ingredients into wet ingredients until well combined.
Transfer batter to pan and drizzle with remaining peanut butter, then dollop with Fluff. Swirl gently with a spoon.
Bake until golden and set, 35 minutes exactly. Let cool completely before slicing, at least 30 minutes.
8x8 pan takes almost double to bake.