1 stick unsalted butter, plus melted butter for brushing
1 tablespoon unsweetened cocoa powder
1/4 cup plus 1 tablespoon all-purpose flour
6 ounces dark chocolate, 70 percent cacao; chopped
1/2 cup granulated sugar
3 large eggs, at room temperature
1 pinch of salt
4 tablespoons marshmallow fluff
Confectioners' sugar, for sprinkling
In a medium saucepan, melt 1 stick of butter with the chocolate over very low heat, stirring occasionally. Let cool slightly.
In a bowl, using an electric mixer, beat the granulated sugar with the eggs and salt at medium-high speed until thick and pale yellow, 3 minutes. Using a rubber spatula, fold in the melted chocolate until no streaks remain. Fold in the 1/4 cup of flour.
Spoon two-thirds of the batter into the prepared ramekins. Using a slightly moistened spoon, divide the marshmallow fluff among the ramekins. Cover with the remaining chocolate batter. Bake in the center of the oven for 16 minutes, until the tops are cracked but the centers are still slightly jiggly. Transfer the ramekins to a rack and let cool for 5 to 8 minutes.