Preheat oven to 375° F. Generously butter, flour, and sugar eight 6-ounce coffee cups or mugs made of ovenproof stoneware (see tip below). Wipe the rims clean.
Place the 8 tablespoons of butter and the chocolate in a medium heatproof bowl over a pan of simmering water and melt, stirring once or twice until smooth. Remove from heat and let cool 5 minutes.
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.
Tip: You can bake these individual cakes in coffee mugs or teacups, or use eight 6-ounce ramekins. (To determine if your mugs are ovenproof, refer to the packaging or check with the manufacturer.) Alternatively, a single cake can be made in a 10-inch springform pan or pie plate with enough regular-size marshmallows to cover. To do this, increase the initial baking time to 22 to 25 minutes or, if you prefer a more gooey center, to 17 to 20 minutes.