Add whole milk, granulated sugar, and unsweetened cocoa powder to a medium saucepan.

Place the saucepan over medium heat and whisk occasionally until the mixture just begins to bubble, being careful not to boil it.

Stir in the bittersweet chocolate and pure vanilla extract, continuing to whisk until the chocolate is fully melted and the mixture is smooth.

Pour the hot chocolate into 4 small mugs and top each with homemade whipped cream.