Measure the milk, heavy cream, cocoa powder, sugar, chopped chocolate, vanilla, and salt so they are ready to use. Finely chop the chocolate if using a bar.

Combine dry cocoa and sugar

In a small bowl, whisk together the unsweetened cocoa powder and granulated sugar to remove lumps and ensure even distribution.

Warm the milk and cream

Pour 3 cups whole milk and 1 cup heavy cream into a medium saucepan. Heat over medium-low heat, stirring occasionally, until the mixture is warm and small bubbles form around the edge (do not boil). This should take about 5–7 minutes.

Add cocoa and sugar slurry

Whisk the cocoa-sugar mixture into the warmed milk and cream until fully incorporated and smooth. Continue stirring and maintain medium-low heat.

Melt in chocolate

Add the 4 oz finely chopped semisweet chocolate (or chips) to the saucepan. Stir constantly until the chocolate is fully melted and the mixture is glossy and smooth, about 2–3 minutes.

Simmer briefly

Reduce the heat to low and let the hot chocolate gently simmer (not boil) for 1–2 minutes to thicken slightly and allow flavors to meld. Stir frequently to prevent scorching.

Finish with vanilla and salt

Remove the saucepan from heat. Stir in 1 teaspoon vanilla extract and a pinch of salt. Taste and adjust sweetness if desired, adding up to 1 additional tablespoon sugar if you prefer it sweeter.

Serve and Garnish

Ladle and top

Divide the hot chocolate among 4 mugs (approximately 1 cup per serving). Top each with a spoonful of whipped cream or a handful of mini marshmallows, or both.

Add final touches

Sprinkle a pinch of ground cinnamon or some chocolate shavings/cocoa powder over the top. If desired, stir in 1 teaspoon of flavored liqueur per cup for an adult version.

Serve

Serve immediately while hot. If making ahead, keep warm over the lowest heat and stir occasionally; reheat gently before serving.