Preheat oven to 375° F. Generously butter, flour, and sugar eight 6-ounce coffee cups or mugs made of oven proof stoneware. Wipe the rims clean.
Place the 8 tablespoons of butter and the chocolate in a medium heat proof bowl over a pan of simmering water and melt, stirring once or twice until smooth.
Using a handheld mixer, beat the eggs, yolk, vanilla, salt, and 1/2 cup sugar at the highest speed until the volume doubles and the mixture becomes foamy, about 5 minutes. Stir the 3 tablespoons of flour into the chocolate, then gradually add it to the egg mixture, beating on low until it's fully incorporated.
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.