1. Heat oven to 350 degrees . Coat three 9-inch round cake pans with nonstick spray for baking.Cake
2. In a medium bowl, whisk together flour, cocoa, baking soda and salt. In a large bowl, with an electric mixer, beat butter until smooth. Add sugar and beat for 2 minutes, until light and fluffy. Beat in eggs, one at a time. On low speed, beat in half the flour mixture, then the milk mixture, followed by remaining flour mixture. Stir in vanilla and divide batter among prepared pans. Bake at 350 degrees for 30 to 35 minutes, or until cake springs back when lightly pressed. Cool in pans on wire racks for 10 minutes. Run a thin knife around edge of pans; turn cakes out of pans and cool completely.Filling
3. While cake layers cool, combine confectioners' sugar, marshmallow cream, butter and 1/2 tsp water in a bowl. Beat on low speed until blended and a good spreading consistency.
4. Once cakes have cooled, place one cake layer on a plate. Spread top with 2/3 cup of the fudge topping. Sprinkle with 1 tbsp of the crushed graham crackers. Spread a second cake layer with half the marshmallow frosting, then invert onto cake layer on platter. Spread top with 2/3 cup of the fudge topping and carefully spread with remaining marshmallow filling (may look marbled). Sprinkle with 1 tbsp crushed graham crackers. Spread remaining layer with remaining 2/3 cup fudge sauce (do not stack onto cake yet). Sprinkle with remaining crushed graham crackers and top with marshmallows.
5. Arrange oven rack so cake layer can be about 3 to 4 inches from heat source (see Note). Heat broiler. Broil marshmallows for 30 seconds, until they just begin to brown. Carefully place layer on cake.
6. NOTE: You may use a creme brulee torch (Bonjour, bedbathandbeyond.com, $30).