Combine the whole milk and 1/2 cup of the heavy cream in a medium saucepan. Warm over medium heat until steam appears and small bubbles form around the edges (do not boil), about 5–7 minutes, stirring occasionally to prevent skin from forming.

Mix cocoa and sugar

While milk warms, whisk the unsweetened cocoa powder and granulated sugar into a small bowl with 1–2 tablespoons of the warm milk to make a smooth paste (this helps prevent lumps).

Melt chocolate

Reduce heat to low. Add the chopped semi-sweet chocolate to the saucepan along with the cocoa paste. Stir constantly until the chocolate is fully melted and the mixture is glossy and smooth, 2–3 minutes. If needed, use a rubber spatula to scrape melted chocolate from the bottom.

Finish and season

Stir in the remaining 2 tablespoons of heavy cream, the 1/2 teaspoon vanilla extract, and a pinch of salt. Taste and adjust sweetness if desired (add up to 1 tsp more sugar). Keep the hot chocolate on very low heat until ready to serve; do not boil.

Portion

Divide the hot chocolate evenly between 4 warmed mugs (roughly 3/4 to 1 cup hot chocolate per mug).

Whipped Cream Topping (Crème Chantilly)

Chill tools

Place a mixing bowl and whisk or beaters in the refrigerator for 10–15 minutes before whipping to help the cream whip faster and hold shape.

Whip cream

Pour 1 cup cold heavy whipping cream into the chilled bowl. Add 2 teaspoons powdered sugar and 1/4 teaspoon vanilla extract. Whip with a hand mixer or whisk until soft peaks form — the cream should be smooth and billowy but not grainy or over-whipped, about 2–3 minutes with an electric mixer (longer by hand).

Top hot chocolate

Spoon or pipe a generous dollop of the whipped cream onto each mug of hot chocolate, creating a tall whipped crown in the Viennois style.

Garnish and Serve

Finish each cup by dusting with a little chocolate shavings or a light sprinkle of cocoa powder. Add a few mini marshmallows if desired. Serve immediately while hot.