In a medium saucepan over medium-low heat, warm whole milk, stirring occasionally until steaming but not boiling, about 3–5 minutes.

Add cocoa powder and sugar to the warm milk and whisk vigorously until dissolved, about 1-2 minutes.

Stir in the finely chopped bittersweet chocolate until melted and mixture is velvety, about 2-3 minutes.

Combine cornstarch with cold water in a small bowl until smooth, then stir into the chocolate mixture while whisking.

Stir constantly for about 2–3 minutes over medium-low heat until the mixture thickens.

Remove from heat and stir in vanilla extract and a pinch of salt until well incorporated.

Pour into mugs and top each serving with whipped cream and desired toppings.