Place egg whites in a large bowl; let stand at room temperature 30 minutes. In a large heavy saucepan, whisk egg yolks, eggs, 1 cup sugar and cornstarch until blended; stir in 4 cups milk. Cook over medium-low heat 10-15 minutes or until mixture is just thick enough to coat a metal spoon and a thermometer reads at least 160°, stirring constantly. Do not allow to boil. Remove from heat immediately. Strain through a fine-mesh strainer into a large bowl., Place bowl in an ice-water bath. Stir occasionally for 5 minutes. Stir in 1-1/2 teaspoons vanilla. Press plastic wrap onto surface of custard. Refrigerate until cold, about 1 hour., For snowballs, add cream of tartar to egg whites; beat on medium speed until foamy. Gradually add remaining sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved. Stir in remaining vanilla. Continue beating until stiff glossy peaks form., In a large heavy skillet, bring remaining milk barely to a simmer over medium-low heat. Working in batches and using two soup spoons, drop meringue by 1/3 cupfuls into milk; poach meringues 4-6 minutes or until firm to the touch, turning once. Using a slotted spoon, remove meringues to paper towels to drain. Repeat with remaining meringue, making a total of 12 snowballs. (Discard remaining milk.) If desired, serve with pecans.