Step 1: In a medium saucepan, whisk together the unsweetened cocoa powder, granulated sugar, and a pinch of salt until well combined.
Step 2: Gradually whisk in 1/2 cup of the whole milk to the dry ingredients until a smooth paste (slurry) forms. This helps prevent lumps.
Step 3: Add the remaining 2 1/2 cups of whole milk and the heavy cream to the saucepan. Place over medium heat and cook, whisking occasionally, until the mixture is hot and steaming, with small bubbles forming around the edges. Do not bring to a rolling boil.
Step 4: Remove the saucepan from the heat. Add the finely chopped dark or semi-sweet chocolate and the vanilla extract. Let stand for 1-2 minutes to allow the chocolate to soften and begin melting.
Step 5: Whisk the hot chocolate vigorously until all the chocolate has completely melted and the drink is smooth, glossy, and well combined.
Step 6: Taste and adjust sweetness if desired. Pour into individual mugs and serve immediately with your favorite toppings like whipped cream, marshmallows, or a dusting of cocoa.