1/4 cup good-quality cocoa powder
1 teaspoon ground espresso powder (optional)
1/2 teaspoon baking powder
1/2 teaspoon fine sea salt
4 ounces (8 tablespoons) unsalted butter
8 ounces good-quality semi-sweet chocolate, coarsely chopped
4 large eggs
1 1/2 cups sugar
2 teaspoons pure vanilla extract
1 1/2 cups (about 7 ounces) marshmallow fluff
1/2 cup salted caramel sauce, store-bought or homemade
1 cup milk chocolate chips or pieces
Preheat oven to 350°F. Grease an 8x8-inch baking dish with cooking spray (or butter), then line with 2 sheets of parchment paper or aluminum foil, allowing a two-inch overhang on each side. Grease inside of paper or foil with additional cooking spray.
In a medium-sized bowl, stir together the flour, cocoa powder, espresso powder, baking powder, and salt, and set aside. Melt the butter and semisweet chocolate together over a double boiler (or in the microwave on medium power for 1 to 2 minutes, stirring every 30 seconds) until the mixture is melted and smooth. In a large mixing bowl, beat the eggs, sugar, and vanilla with an electric mixer on high speed until light and fluffy, about 2 minutes. Fold the melted butter and chocolate into the egg mixture, then gently fold in the dry ingredients until just combined and no flour more is visible.
Dollop one-half of the brownie batter into the prepared baking dish and smooth with an off-set knife or silicone spatula. Drop dollops of half the marshmallow cream and caramel on top of the batter. Top with the remaining brownie batter and carefully smooth to fill the pan. Finally, drop dollops of the remaining marshmallow cream and caramel over the top. Drag the offset knife through the batter in both perpendicular and horizontal directions, making sure to reach the bottom of the pan. Finish by sprinkling the milk chocolate chips over everything.
Bake the brownies for 28 to 30 minutes, until only a few moist crumbs are visible when the brownies are pricked with a toothpick.
Let the brownies cool in the pan on wire rack for 1 hour. Using foil overhang, lift brownies out of pan. Let the brownies cool completely before serving, preferably overnight. Carefully peel off the foil and put the brownie on a large cutting board; cut into squares. Drizzle extra caramel on top, if desired.