Preheat the oven to 375° Fahrenheit. Brush 5 4-ounce ramekins with melted butter. In a small bowl, whisk the cocoa powder with 1 tablespoon of the flour; dust the ramekins with the cocoa mixture, tapping out the excess. Transfer the ramekins to a baking sheet, and set aside.
In a medium bowl over a pot of simmering water, melt 1 stick of butter with the chocolate, stirring occasionally until fully melted. Remove from heat and let cool slightly.
In a bowl of an electric mixer fitted with the whisk attachment, beat the granulated sugar, eggs, egg yolks, and salt over medium-high speed until thick and pale yellow, about 3-4 minutes. Add the vanilla extract, and using a rubber spatula, fold in the melted chocolate until no streaks remain. Gently fold in the flour until mostly incorporated. Be sure not to over mix.
Spoon the batter into the prepared ramekins. Do not fill the jars up to the rim or the cake will spill over when it begins to rise.
Bake cakes in the center of the oven for 10-14 minutes, until the tops are cracked but the centers are still slightly jiggly. Transfer the ramekins to a rack and let cool for about 5 minutes.
Run the tip of a small knife around each cake to loosen. Invert a small plate over each cake and, using pot holders, or a clean dishtowel, invert again. Carefully lift off the ramekins.
Cut marshmallows in half and arrange 3-4 marshmallows on top of each molten chocolate cake. Torch the marshmallows lightly until roasted. Alternatively, you can roast the marshmallows in your oven (broil setting) and then gently place the marshmallows on the molten chocolate cakes.
Serve immediately.