Ladle the batter into each cup until it's about 1/2 inch from the rim. Bake until the cakes puff and begin to crack but the centers are still a bit runny, 13 to 17 minutes. Remove from oven. Sprinkle with marshmallows, then return to oven for 2 to 4 minutes, until the marshmallow tops begin to crisp. Remove from oven; let cool about 5 minutes. Sift the cocoa on top.