Step 1Line a 9-inch pie plate with the poundcake slices, very slightly overlapping. Through plastic wrap, flatten the cake with your hands to compact it into the plate. Trim the top edges as needed. Freeze the cake until solid, about 3 hours.
Step 2Spoon the ice cream into the frozen cake shell. Smooth the top, shaping it into a dome. Freeze until hard, 3 hours.
Step 3Beat the egg whites, sugar and cream of tartar in the bowl of an electric mixer to stiff peaks. Spread the meringue over the hard ice cream; swirl to make a decorative pattern. Alternately, spread a 1/2-inch thick layer of meringue over the hard ice cream; smooth with a long spatula. Then spoon the remaining meringue into a pastry bag with a large (#7) star tip; decorate the top with large rosettes.
Step 4Meanwhile, place a rack about 7 inches under the broiler element and heat the broiler.
Step 5Broil until the meringue is golden, 5 to 6 minutes. (You can also use a propane torch.) Freeze until frozen, 2 to 3 hours. (The pie can be kept frozen, covered loosely with plastic wrap, up to 5 days.)
Step 6To serve, remove the pie from the freezer 20 minutes before serving. Cut it into wedges. Pass the Warm Bittersweet Sauce separately.