Spray a 7 3/8 x 3 3/8 x 2 1/4-inch loaf pan with nonstick cooking spray and line bottom and sides of pan with wax paper.
Pack frozen dessert into prepared pan, cover with plastic wrap, and freeze until firm.
In small bowl, using electric mixer, beat egg whites with cream of tartar until soft peaks form; gradually add sugar and continue beating until egg whites are stiff but not dry.
Remove frozen dessert from pan onto a flameproof serving plate; remove wax paper.
Arrange graham crackers along sides and over top of loaf; spread beaten egg whites over entire surface of loaf and broil until egg whites are browned, 1 to 2 minutes.