Heat oven to 275°F. Line cookie sheet with cooking parchment paper. In large bowl, beat egg whites and cream of tartar with electric mixer on medium speed until foamy. On high speed, beat in 1 tablespoon of the granulated sugar at a time until stiff glossy peaks form and sugar is almost dissolved, about 6 minutes. Spread meringue to form 10x10-inch heart shape on cookie sheet, building up edges with back of spoon.