Make and bake your brownie batter according to the box instructions and allow them to cool completely.
Once cool enough to remove from the pan, place on a cutting board or wire rack then prepare your marshmallows
On low, in a microwave safe bowl, melt marshmallows until soft and gooey; it should take about 20-30 seconds. They will likely still hold their shape, and you can stir them up and they look meltier.
Once cooled enough to handle, using your fingertips, grab some marshmallow and slowly spread your hands and fingers apart to create a web design (note: you may want to lightly grease your fingertips to allow the marshmallow to come away from your hands easier).
Repeat until you have your desired look.
Notes
Marshmallow Tip: Spread the marshmallow from edge to edge, and pull down, to use the sides to break away from sticking to your fingers.
Marshmallows:The marshmallows should be warm and stretchy, not piping hot. If the marshmallow becomes too firm, warm it for a few more seconds in the microwave.
Work Quickly: Work quickly, as the marshmallow becomes harder to stretch as it cools.
Clean Slices: For cleaner slices, let the webs set and then grease your knife between cuts.
Styling Tip: Cut into individual brownies prior to, for easier slicing, and to pull webs down the sides of each piece.
Nutritional Info: Will vary based on how the brownies are prepared and how much marshmallow used.