In a medium saucepan set over medium heat, combine 4 cups whole milk and 1/2 cup heavy cream. Warm the mixture until it is steaming and small bubbles form around the edges but do not let it boil (about 5–7 minutes). Stir occasionally to prevent a skin from forming and to distribute heat evenly.

Incorporate cocoa and sugar

Whisk in 2 tablespoons unsweetened cocoa powder and 2 tablespoons granulated sugar until fully dissolved and no lumps remain. Continue heating for 1–2 minutes to bloom the cocoa.

Melt the chocolate

Reduce heat to low. Add 6 ounces finely chopped bittersweet chocolate to the warm milk mixture and whisk constantly until the chocolate is completely melted and the mixture is smooth and glossy (about 2–3 minutes).

Season and finish

Stir in a pinch of sea salt, 1 teaspoon vanilla extract, and 1/4 teaspoon ground cinnamon (if using). Taste and adjust sweetness — add up to 1 tablespoon more sugar if you prefer it sweeter. Keep the hot chocolate on low for 1 more minute to meld flavors, but do not boil.

Serve

Ladle the hot chocolate into 4 warmed mugs (about 1 cup each). Top each serving with marshmallows or a generous dollop of whipped cream, and garnish with mini chocolate shavings or a light dusting of cocoa powder. Serve immediately.

Storage and reheating

If you have leftovers, let the hot chocolate cool to room temperature, then refrigerate in an airtight container for up to 3 days. Reheat gently on the stovetop over low heat, whisking to recombine, and add a splash of milk if it has thickened.