Pour 4 cups whole milk into a medium saucepan. Warm over medium-low heat until steam rises and small bubbles form around the edges (do not boil), about 5–7 minutes. Stir occasionally to prevent a skin from forming.

Mix cocoa and sugar

While the milk is warming, in a small bowl combine 4 tablespoons unsweetened cocoa powder and 4 tablespoons granulated sugar. Add 2–3 tablespoons of the warm milk to the cocoa-sugar mixture and whisk to make a smooth paste (this prevents lumps).

Combine and dissolve chocolate

Stir the cocoa paste back into the saucepan with the warm milk. Add 4 ounces finely chopped bittersweet chocolate and a pinch of fine salt. Continue to cook over medium-low heat, whisking constantly until the chocolate is fully melted and the mixture is smooth and slightly thickened, about 2–3 minutes. Do not let it boil.

Finish flavor

Remove the saucepan from heat and whisk in 1 teaspoon vanilla extract. Taste and adjust sweetness if needed by adding up to 1 additional tablespoon sugar, whisking until dissolved.

Optional Finishing & Garnish

Make whipped cream (optional)

If making fresh whipped cream, chill a mixing bowl and beaters briefly. Pour 1 cup heavy cream into the chilled bowl, add 1 tablespoon powdered sugar, and whip on medium-high until soft peaks form, about 2–3 minutes. Keep chilled until ready to use.

Assemble and serve

Ladle hot chocolate into 4 mugs (about 1 cup each). Top each with a dollop of whipped cream or a few spoonfuls of store-bought whipped cream, sprinkle mini marshmallows (about 2 tablespoons per mug), dust with a pinch of cocoa powder or chocolate shavings and a light sprinkle of ground cinnamon or place a cinnamon stick for stirring. Serve immediately.