Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly and perfectly.
Grease your 9x13 inch baking pan with a bit of butter or line it with parchment paper for easy removal later.
In a large mixing bowl, combine the Betty Crocker Fudge Brownie Mix, white sugar, half and half, and melted butter. Use a whisk or spatula to mix until just combined. Avoid overmixing for the fudgiest texture.
Gently fold in the chocolate chips into the brownie batter. This will create delicious pockets of melted chocolate when baked.
Pour the brownie batter into the prepared baking pan. Use a spatula to spread it evenly across the pan.
Place the pan in the preheated oven and bake for about 25-30 minutes. The brownies should look set but still slightly underbaked in the center.
Remove the brownies from the oven and immediately sprinkle the marshmallows on top. Return the pan to the oven and bake for an additional 5-10 minutes, or until the marshmallows are golden and gooey.
Allow the brownies to cool in the pan on a cooling rack. Once cooled, slice into squares and serve.