Prepare brownies according to package directions; stir in ½ cup walnuts. Transfer to a greased 13x9-inch baking pan. Bake according to package directions. Let cool completely.
Spread softened ice cream over brownie. Freeze until ice cream is hard.
Spread whipped topping over ice cream. Drizzle warmed hot fudge over frozen whipped topping; sprinkle with chopped walnuts. Freeze until ready to serve.
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