Step 1: In a medium saucepan, whisk together the cocoa powder, granulated sugar, and salt until well combined. Add about 1/2 cup of the whole milk to this mixture and whisk thoroughly to form a smooth paste, ensuring there are no lumps.
Step 2: Gradually whisk in the remaining whole milk and the heavy cream into the saucepan. Place the saucepan over medium-low heat.
Step 3: Heat the mixture, stirring frequently, until it is hot and steaming but not boiling. Be careful not to let it come to a rolling boil, as this can scorch the milk.
Step 4: Remove the saucepan from the heat. Add the finely chopped dark chocolate and vanilla extract. Let it sit for 1-2 minutes to allow the chocolate to melt.
Step 5: Whisk the hot chocolate vigorously until the chocolate is fully melted and the mixture is smooth and velvety. Taste and adjust sweetness if desired.
Step 6: Pour into mugs and top with your favorite additions like whipped cream, marshmallows, or chocolate shavings. Serve immediately and enjoy!