Prepare the Peanut Butter Filling: In a medium bowl, combine 1 cup of creamy peanut butter, 1 cup of powdered sugar, 2 tablespoons of unsalted butter, 1 teaspoon of vanilla extract, and a pinch of salt. Mix until smooth and creamy.

Grease & Preheat: Preheat oven to 350°F (180°C) and grease a 9x13-inch baking dish, lining it with parchment paper.

Mix Dry Ingredients: In a small mixing bowl, whisk together 1 cup of all-purpose flour, 1/2 teaspoon of baking powder, and 1/4 teaspoon of salt.

Mix Wet Ingredients: In a large mixing bowl, whisk together 3 large eggs, 1 cup of granulated sugar, 1/2 cup of brown sugar, 1/2 cup of canola oil, and 2 teaspoons of vanilla extract until smooth.

Melt Butter & Chocolate: In a medium saucepan over low heat, combine 1 cup of chopped bittersweet chocolate and 1/2 cup of unsalted butter. Stir until melted and smooth.

Combine Mixtures: Pour the melted chocolate mixture into the wet ingredients and whisk until fully combined. Gradually fold in the dry ingredients.

Pour & Swirl: Spread the brownie batter evenly into the greased pan. Dollop spoonfuls of peanut butter filling and marshmallow fluff on top, swirling gently.

Bake: Bake for 26 to 32 minutes, checking doneness with a toothpick.

Cool & Serve: Once baked, cool in the pan, lift out using parchment, cut into squares, and serve.