Preheat oven to 375 degrees F. Using the butter, grease 6 (8-ounce) oven-proof, straight-sided coffee cups or ramekins, and sprinkle each with 1 teaspoon sugar. Place prepared coffee cups or ramekins in the refrigerator while you prepare the batter.
In a large metal bowl, combine the cocoa powder and 1/2 cup of sugar. Heat milk in a small saucepan, just until scalded. Pour milk over cocoa and whisk until smooth and sugar is dissolved. Whisk in egg yolks and vanilla.
In a mixing bowl, beat the egg whites with remaining 1/4 cup of sugar until stiff and glossy. Fold the egg whites into the chocolate mixture until thoroughly blended.
Pour the chocolate mixture into the chilled coffee cups or ramekins. Place about 5 marshmallows into the center of each ramekin just before baking.
Bake souffles until they are puffed and somewhat firm, about 18 to 20 minutes. Remove from the oven and serve immediately with chocolate sauce and dust with confectioners' sugar and garnish with mini marshmallows.
Yield: 6 servings