Prepare the brownies according to the directions on the box. Pour the batter into the prepared pan, using a rubber spatula to spread the brownie batter into an even layer and bake until a knife inserted into the middle comes out mostly clean. Set aside to cool completely.
Once the brownies are fully cool, place the marshmallows in a a microwave safe bowl and microwave until fully melted. Let the marshmallows cool for a minute until tacky.
Pick the marshmallow up between your fingers and stretch the marshmallows out to form long strings that look like spiderwebs, pulling them over the brownies and pressing down onto the brownies. Repeat until the brownies are covered in “spider webs”. Top with sprinkles and enjoy!