Preheat the oven to 450 degrees F. Grease and sugar 4 (6-ounce) ramekins or baking dishes with the 1 1/2 teaspoons of butter and 1 teaspoon of sugar in each, tapping out the excess flour. Set on a baking sheet.In a double boiler, or a metal bowl set over a pan of simmering water, melt the remaining 1/4 pound of butter with the chocolate and cream, stirring until smooth. Remove from the heat.

In a bowl, whisk together the eggs, egg yolks, sugar, vanilla, and salt until thick ribbons form, about 5 minutes. Sift 2 tablespoons flour into the egg mixture and fold together. Fold the chocolate into the egg mixture.

Spoon two-thirds of the batter into the prepared ramekins. Using a wet spoon, divide the marshmallow fluff between the ramekins. Cover with the remaining chocolate batter. Bake in the oven for 14 minutes, until the tops are cracked but the center is still slightly jiggly. Transfer the ramekins to a rack  and let cool for 5 minutes.

To unmold the cakes, run the tip of a paring knife around each cake to loosen. Invert a small plate over each cake, and invert the cake again onto the serving plate. Dust the cakes with confectioners' sugar and serve immediately.