Whisk together cocoa powder, granulated sugar, and salt in a small saucepan.
Add water and whisk until the dry ingredients come together in a smooth paste.
Warm the pan over medium heat, whisking, until it comes to a boil. Let the mixture boil for one minute, continuing to whisk as it bubbles and thickens slightly.
Pour in the milk and warm the cocoa to the desired temperature, making sure to remove from heat before it comes to a boil.
Off heat, stir in vanilla extract and serve. Top as desired.