Preheat oven to 450° F. Grease and flour 2 (6 oz) ramekins using butter and flour in each, tapping out the excess flour. Set on a baking sheet.
In a double boiler, or glass or metal bowl set over a pan of simmering water, melt the butter with the chocolate, cream, and powdered sugar, stirring until smooth. Remove from heat.
In a bowl, whisk together the egg, egg yolk, sugar, vanilla and salt until thick ribbons form, about 3 minutes. Sift 1 tablespoon flour into the egg mixture. Fold the chocolate into the egg mixture. Divide between the ramekins and bake until the sides of the cake are set and the tops are puffed but still soft, approximately 11-14 minutes.