Preheat oven to 350°F and prepare an 8x8" pan by greasing or with parchment.
Heat butter either in the microwave or in a sauce pan on the stove, stir in granulated sugar and brown sugar with a spatula.
Stir in eggs one at a time into the wet ingredients, making sure eggs are fully incorporated.
Add black cocoa to the wet ingredients until combined.
Add flour and salt into wet ingredients and stir in with a spatula until just combined.
Pour batter into prepared pan. Bake brownies on middle rack of oven for about 28-30 minutes. I checked mine at 25 minutes and the middle was not set, but was set about 3 minutes later, which is what you want.
When brownies come out of the oven, let sit out for about 10 minutes, and then move pan to refrigerator to chill the rest of the way.
When brownies are sufficiently chilled, place marshmallows in a microwave safe bowl. Heat for about 15 seconds, stir, and then microwave for about another 10-15 seconds.
Stir and then let marshmallows cool for about 1 minute. Pick up a dollop of marshmallow and spread between fingers creating a web pattern and spread/lay over brownies. Repeat until you have covered the brownies.