Combine sugar and cocoa in a slow cooker; stir in milk. Cover and cook on low setting for 3 to 4 hours. At serving time, stir in vanilla.
With a whisk or a hand mixer, carefully beat until frothy. Ladle hot cocoa into mugs. Garnish with marshmallows or whipped cream.
Nice & Spicy Cocoa: Add one teaspoon cinnamon and 1/8 teaspoon nutmeg along with the cocoa. Cover and cook as directed.
Mocha Cocoa: Prepare recipe as directed. At serving time, stir 3/4 teaspoon instant coffee granules into each mug of hot cocoa; stir to mix.