To prepare cake, combine 3/4 cup boiling water and 1/2 cup cocoa, stirring until blended. Cool. Place granulated sugar, 1/2 cup softened butter, and 1 teaspoon vanilla in a large bowl; beat with a mixer at medium speed until blended. Add cocoa mixture and egg substitute; beat well. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking soda, and 1/4 teaspoon salt in a bowl, stirring well. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture. Spoon batter into a 13 x 9-inch baking pan coated with cooking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Top with marshmallows. Bake an additional 2 minutes or until marshmallows are soft.
To prepare frosting, combine 1/4 cup cocoa, evaporated milk, 3 tablespoons melted butter, and 1/8 teaspoon salt in a medium, heavy saucepan over medium heat. Cook for 4 minutes, stirring frequently. Stir in powdered sugar and 1 teaspoon vanilla. Cook 2 minutes, stirring constantly. Drizzle frosting over cake. Cool. Cut cake into squares.
These were quite yummy and got rave reviews from my friends. I made as cupcakes (made one dozen) - baked at 325 for the full 20 minutes. Moist, not crumbly, easy to make; I will certainly make these again.
Tasty, easy, and good-looking dessert! Does not require much prep time but finished product looks like it took all afternoon. Had some problems with the bottom chocolate cake layer deflating while cooling so the entire cake sunk into the pan.