Grease a 7-inch bowl (large enough to hold 1/2 gallon ice cream, but not wider than your cake round!) with flavorless oil (see note above) or unsalted butter.
Let ice cream soften until it is workable. Pack tightly in the bowl. Return bowl to freezer until it hardens.
When ready to assemble, beat egg whites until foamy.
Add tartar and whip until stiff peaks form. Gently fold in powdered sugar, a little bit at a time.
Place frozen layer cake on a plate slightly larger than the cake. Unmold the ice cream by centering it on the cake. If the ice cream does not release from the bowl, see notes.
Do not trim the cake. "Frost" cake and ice cream with meringue, covering completely, creating a decorative pattern by forming peaks with a spoon or spatula.
Bake in preheated at 450 degrees for 6-7 minutes until meringue peaks are golden brown.