In a medium saucepan over medium heat, combine cocoa powder, sugar, water, and salt.
Cook until smooth, stirring constantly, and bring to a simmer.
Reduce the heat, and add the milk, making sure it doesn't boil.
Remove from the heat and stir in the vanilla. Pour the hot chocolate into mugs and serve immediately with your favorite toppings.
Notes
For one: in the bottom of a mug, mix 2 teaspoons of cocoa powder and 2 tablespoons of sugar. Heat 1 ¼ cups of milk in the microwave or stovetop until hot (but not boiling). Pour a little bit of the hot milk, just enough to cover the powdered mixture, and with a small whisk or spoon, dissolve the cocoa and sugar. Once you have a chocolatey base, add ¼ teaspoon of vanilla. Pour the rest of the hot milk into the mug and stir to combine.