1. Preheat oven to 400°F. In 9-inch pie plate, with hand, mix walnuts, brown sugar, and 3 tablespoons butter or margarine. Press mixture onto bottom and up side of pie plate. Bake 8 minutes. Cool crust completely on wire rack. Meanwhile, place vanilla ice cream in refrigerator to soften slightly.
2. Evenly spread softened vanilla ice cream in crust; freeze until firm, about 1 1/2 hours.
3. In 2-quart saucepan over medium heat, cook cocoa, heavy cream, 1/4 cup sugar, and 4 tablespoons butter or margarine (1/2 stick), stirring constantly, until mixture is smooth and boils. Remove saucepan from heat; stir in vanilla extract. Cool fudge sauce to room temperature.
4. Pour fudge sauce over ice-cream layer. Return pie to freezer; freeze until fudge sauce hardens, about 30 minutes.
5. Remove chocolate ice cream from container to medium-sized bowl; let stand at room temperature, stirring occasionally, until smooth spreading consistency but not melted. Spread chocolate ice cream over fudge. Return pie to freezer; cover and freeze until firm, at least 3 hours.
6. About 20 minutes before serving, preheat oven to 500°F. Prepare meringue: In small bowl, with mixer at high speed, beat egg whites, salt, and cream of tartar until soft peaks form; beat in 1/2 cup sugar, 2 tablespoons at a time, beating until sugar completely dissolves and whites stand in stiff, glossy peaks. With spoon, quickly spread meringue over top of pie, sealing to edge and swirling meringue with back of spoon to make attractive top. Bake 2 to 3 minutes, until meringue is lightly browned.