Warm the milk. Add milk to a small saucepan and set over medium-low heat.
Warm until steaming and small bubbles form around the edges. Do not let it boil.
Bloom the cocoa. In a mug or small bowl, mix cocoa powder, sugar, and a pinch of salt. Stir in 2 to 3 tablespoons of the warm milk to make a smooth paste with no lumps.
Combine and heat. Whisk the cocoa paste into the saucepan of warm milk.
Keep whisking until fully combined and smooth.
Add chopped chocolate. Stir in the chopped chocolate and whisk gently until melted and glossy. This deepens the flavor and thickens the drink slightly.
Finish with vanilla. Turn off the heat and stir in the vanilla. Taste and adjust sweetness or salt.
If it’s too thick, add a splash more milk.
Serve. Pour into a warm mug and add your favorite toppings. Enjoy right away.