Preheat oven to 375ºF. Mist 4 6-oz. ramekins with cooking spray and place on a baking sheet.
Combine butter and chocolate in a large bowl and microwave on high until almost melted, 1 to 2 minutes. Stir until smooth and set aside to cool. Stir in flour.
With an electric mixer on medium-high speed, beat sugar, eggs, vanilla and salt until lightened and smooth, about 5 minutes. Fold in chocolate mixture.
Divide batter among ramekins; bake until cakes have risen and are cracked on top but still wet in centers, 12 to 15 minutes. Lightly press a marshmallow on top of each cake and bake until marshmallows have melted and begin to brown, 2 to 3 minutes longer. Place ramekins on a wire rack for 5 minutes to cool before serving.