The secret to really good sugar free hot chocolate is to make it so good it doesn't even need sweetener! The trick is to use the very best ingredients.
Add the cocoa powder and water to a small pot. Heat over medium heat for a minute or two, whisking continuously, until the mixture forms a thick, smooth, and glossy paste.
Add 1/4 cup of milk and whisk until the mixture is completely smooth again. Add another 1/4 cup of milk and whisk again until smooth. Add the remaining 1/2 cup of milk and whisk until smooth.
Stir in the vanilla and a generous pinch of salt. Heat the hot chocolate until is is just barely steaming, but do not let it simmer or boil. Add sugar free sweetener to taste, if desired. Makes two 1/2 cup servings, or the equivalent of four demitasse cups.