Cut the sheetcake into an 8" circle and center on a baking sheet. Sprinkle the cake with Grand Marnier (3 tablespoons) soaking it well!
Mound scoops of ice cream onto the cake creating a dome shape. Try to leave a 1" border around the edge of the cake. Freeze until solid, several hours or overnight.
Preheat your oven thoroughly to 450 degrees.
In a stand mixer, whip the egg whites and cream of tartar until they form soft peaks. Stir in the vanilla. Slowly begin adding the sugar continuing until stiff peaks form. Spread over the ice cream hiding the frozen cake. If you like, swirl decoratively with the back of a spoon or place some in a pastry bag and pipe on a decorative edge. Gently press an eggshell half into the top of the works. Immediately place in the hot oven and briefly bake until the meringue begins to turn a light brown colour, about 5 to 8 minutes.
Sit all your guests back down! Gently warm the remaining Grand Marnier in a small pot just until small bubbles begin to appear. Pour some into the eggshell, dim the lights and ignite. At the table you may pour some more liqueur from a ladle into the eggshell and it too will ignite.