For brownie, grease a 15x10x1-inch baking pan and line with waxed paper; grease and flour the waxed paper. Set pan aside. Prepare brownie mix according to package directions; pour batter into prepared pan. Bake in a 350 degree F oven for 20 minutes. Cool in pan on a wire rack for 10 minutes; loosen edges and remove from pan. Cool completely on a wire rack.
Meanwhile, for filling, heat milk in a saucepan over low heat until steaming, stirring constantly. Add marshmallows; stir until melted. Add broken chocolate bars; stir until melted. Remove from heat. Cover surface of chocolate mixture with waxed paper; cool to room temperature.
In a chilled bowl beat the 1 cup whipping cream with chilled beaters on medium speed until soft peaks form (tips curl). Fold cooled chocolate mixture into whipped cream. Line two 7-1/2 x 3-1/2x2-inch loaf pans with plastic wrap. Cut brownie to fit the long sides and the bottom of pans. Line pans with brownie pieces, cutting to fit. (Do not line ends of pans with brownie.) Fill lined pans with cooled filling. Cover with additional brownie pieces. Cover with plastic wrap; freeze overnight.
To serve, uncover and invert terrine onto a serving platter. Remove plastic wrap. Use a hot knife to slice terrine. Place 2 slices on each dessert plate. If desired, serve with hot fudge sauce or chocolate sauce. Makes 12 to 14 servings.