Heat 4 cups of the milk in a large saucepan over medium heat until steaming.
Whisk the sugar, cornstarch and 1/2 teaspoon salt in a large bowl.
Whisk in the egg yolks and the remaining 1/2 cup milk.
Whisk half of the hot milk into the egg mixture until smooth, then gradually whisk the egg-milk mixture into the saucepan. Cook over medium heat, whisking constantly, until the mixture boils. Continue to cook, whisking constantly, until it has thickened to a puddinglike consistency, 3 to 4 minutes.
Remove from the heat and stir in the vanilla.
Cool slightly, stirring a few times to prevent a skin from forming.
Place a piece of plastic wrap directly on top of the pudding and refrigerate until completely cooled and thickened, about 4 hours.
Before serving, whisk the pudding vigorously until smooth and creamy.
Cookies and Cream: Crush 20 chocolate wafer cookies, stir some into the pudding and top with the rest.
Fresh Berries and Gingersnaps: Fold in 2 cups of fresh, mixed berries and top with about 12 finely crushed gingersnaps.
Vanilla-Banana: Fold in 3 sliced bananas. Cover and refrigerate for 1 hour. Fold in 24 crushed vanilla wafer cookies and top with more sliced bananas.
Rocky Road: Fold in 1 cup each mini marshmallows and toasted chopped walnuts and 1/2 cup of mini chocolate chips.
Sprinkle with more chocolate chips.
Peanut Butter* and Jelly (See Cook's Note): Fold in 1 cup quartered red grapes and 1/3 cup each chopped roasted peanuts and peanut butter chips. Top with whole grapes.
