Coat 6 (10-ounce) custard cups evenly with cooking spray. Place cups in a baking pan.
Stir together 2/3 cup sugar and next 4 ingredients in a large bowl until blended.
Beat egg yolks at medium speed with an electric mixer until thickened and pale. Add 1 1/2 cups milk, beating until blended. Stir in sugar mixture, beating until blended.
Beat egg whites at medium speed with an electric mixer until foamy. Add cream of tartar, beating until soft peaks form. Add 3 tablespoons sugar, and beat until stiff. Gently fold into egg yolk mixture. Spoon batter evenly into prepared custard cups, filling three-fourths full. Fill baking pan with hot water to a 1-inch depth.
Bake at 350° for 30 minutes or until cakes are set and tops are golden. Remove cups to wire racks to cool. Serve warm or chilled. Garnish, if desired.