Preheat oven to 350°. Grease a 13 by 9 inch baking pan.
Melt 4 oz (4 squares) unsweetened chocolate and 2 sticks butter in a 2-quart bowl in the microwave on high for 3 minutes. Stir until the chocolate is completely melted. Add the eggs, 2 cups sugar, and 1 teaspoon vanilla extract and mix well with a spatula. Add the flour and stir to combine. Spread the batter evenly in the prepared pan. Bake for 25 minutes.
While the brownies are baking, make the frosting. Melt 4 oz (4 squares) unsweetened chocolate and 2 sticks softened butter in a medium bowl in the microwave on high for 3 minutes. Stir until the chocolate is completely melted. Add 1/2 cup egg substitute, 1 teaspoon vanilla extract, and 1 (1 lb) box sifted confectioner's sugar and stir with a spoon until smooth. Stir in the marshmallows; they will soften but not melt completely. Spread the frosting over the warm brownies. The frosting will set up when the brownies are completely cooled. When cool, cut into 1 1/2-inch squares and store in the refrigerator in a plastic container with a snap-on lid.