In a medium saucepan set over medium heat, combine 3 cups whole milk and 1 cup heavy cream. Warm gently, stirring occasionally, until mixture is steaming and small bubbles form around the edges but not boiling (about 5–7 minutes).

Whisk cocoa and sugar

While milk warms, whisk together 1 tablespoon unsweetened cocoa powder and 2 tablespoons granulated sugar in a small bowl (this helps prevent lumps).

Incorporate cocoa mixture

When milk is steaming, whisk the cocoa-sugar mixture into the saucepan, stirring constantly to fully dissolve and evenly distribute the cocoa.

Melt chocolate

Add 4 ounces finely chopped bittersweet chocolate to the hot milk mixture. Reduce heat to low and stir constantly until chocolate is fully melted and the mixture is smooth (2–3 minutes). Avoid boiling.

Season and finish

Remove the pan from heat. Stir in 1 teaspoon vanilla extract and a pinch of fine salt. Taste and adjust sweetness if needed (add up to 1 tablespoon more sugar). If you want peppermint hot chocolate, stir in 1/4 teaspoon peppermint extract now.

Serve & Garnish

Divide the hot chocolate among 4 mugs (about 1 cup each).

Top each mug with a generous dollop of whipped cream (optional) and a handful of marshmallows if desired.

Sprinkle with ground cinnamon and/or chocolate shavings or a light dusting of cocoa powder. Serve immediately.