Soften the frozen yogurt in the refrigerator for 15 to 20 minutes. Meanwhile, with a serrated knife, slice the cake horizontally 1 inch from the top. Carefully remove the top of the cake; set aside. Cut and scoop out a tunnel around the center of the cake, leaving a 3/4-inch border of cake on the sides and bottom. (Reserve the scooped-out cake for trifle or another use.) Spoon the softened frozen yogurt evenly into the tunnel, replace the top of the cake, wrap with plastic wrap, and freeze for about 2 hours, or until firm. When ready to serve, dollop the cake with the whipped topping.

ice cream and, for the finishing touch, garnish with fresh summer berries and maybe a few mint sprigs. You can freeze the cake for up to 3 days - just add the whipped topping and any garnishes right before serving.