Bake the brownies a week or two ahead of time. Cover and freeze until the day you want to make the dessert. Thaw to room temperature (about 1 hour), and continue with step 2.
Chill the bowl and beaters about 20 minutes in the refrigerator before whipping chilled cream. Beat the cream only until stiff. Overbeating or beating on low speed instead of on high can cause the cream to separate into fat and liquid.
No chocolate topping? Place 2 tablespoons chocolate chips and 1/2 teaspoon shortening in a microwavable bowl. Heat on High 30 to 45 seconds; stir. Drizzle over frosting.