In a medium bowl, combine creamy peanut butter, powdered sugar, unsalted butter, and a pinch of salt. Use a hand mixer or spatula to mix until smooth and well-blended.

Adjust your oven rack to the center position and preheat to 350°F (180°C). Grease a 9x13 inch baking pan and line it with parchment paper.

In a small bowl, whisk together all-purpose flour, baking powder, and salt until well combined.

In another large bowl, whisk together room temperature eggs, granulated sugar, brown sugar, canola oil, and pure vanilla extract until the mixture appears thick and pale.

In a small saucepan over low heat, melt chopped bittersweet chocolate and unsalted butter together. Stir frequently until completely melted and smooth, then incorporate cocoa powder.

Add the melted chocolate mixture to the egg mixture and whisk until fully combined. Carefully fold in the dry ingredients with a spatula.

Pour the brownie batter into the lined baking pan and spread it evenly. Dollop the prepared peanut butter filling over the brownie batter, then top with spoons of marshmallow fluff.

Place the assembled brownies in your preheated oven and bake for 26-32 minutes. Insert a toothpick into the center.

Once baked, remove the pan from the oven and allow it to cool completely on a wire rack for about an hour. Cut into squares to serve.