In a small saucepan, whisk together the cocoa powder, sugar, and salt until no lumps remain.
Gradually add about 1/2 cup of the milk while whisking continuously to form a smooth paste.
Pour in the remaining milk and stir well.
Cook over medium-low heat, stirring frequently, until the mixture is hot but not boiling (about 5 to 7 minutes).
Remove from heat and stir in the vanilla extract and chopped chocolate (if using), until fully melted.
Taste and adjust sweetness or chocolate intensity as needed.
Pour into mugs and serve immediately.