Preheat oven to 350°.
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.