Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly and come out perfectly gooey.
In a medium saucepan over low heat, melt the 1 1/2 sticks (3/4 cup) salted butter along with the 1 cup semi-sweet chocolate chips. Stir until smooth, then remove from heat and let it cool for a few minutes.
In a large mixing bowl, whisk together the melted chocolate mixture with the 3/4 cup granulated sugar, 2 teaspoons vanilla extract, and 2 teaspoons instant coffee granules. Add the 3 large eggs, one at a time, mixing well after each addition.
Sift in the 1/3 cup all-purpose flour, 1/3 cup unsweetened cocoa powder, and 3/4 teaspoon baking powder. Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Be careful not to overmix!
Gently fold in the 2-4 squares of broken graham crackers and half of the 1 (4.4 ounce) Hershey chocolate bar pieces into the brownie batter.
Grease your cast-iron skillet lightly with butter or cooking spray. Pour the brownie batter into the skillet, spreading it evenly.
Sprinkle the remaining pieces of the Hershey chocolate bar on top of the brownie batter. Then, spoon the 1 cup marshmallow fluff over the top, spreading it evenly. If you’re using marshmallows, place them on top in a single layer.
Place the skillet in the preheated oven and bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs.
Allow the skillet brownie to cool for about 10 minutes. Serve warm, and enjoy the gooey goodness straight from the skillet!
