1 stick (4 oz.) unsalted butter, plus more for greasing
1/4 cup sugar, plus more for coating
4 oz. bittersweet chocolate, chopped
2 large eggs, plus 2 large yolks, at room temperature
1 tbsp. flour
3 tbsp. heavy cream
2/3 cup marshmallow cream
1 graham cracker sheet (2 squares), finely chopped
Preheat oven to 425. Grease four 6-ounce custard cups, then coat with sugar; place on baking sheet. In a medium bowl, microwave butter and chocolate at medium power for 2 minutes. Stir until smooth.
In a medium bowl, whisk together eggs, yolks and sugar until foamy. Whisk in the chocolate mixture, then flour. Fill the prepared custard cups 2/3 full with batter. Bake until edges are puffy, 13 to 15 minutes.
Meanwhile, in a medium bowl, microwave heavy cream at high for 30 seconds. Stir in marshmallow cream until smooth.
Carefully turn cakes onto plates and drizzle marshmallow sauce on the side. Top with graham cracker crumbs.