In a small bowl, whisk together the unsweetened cocoa powder and 1/4 cup of the cold milk until smooth and lump-free to create a cocoa slurry. This prevents clumps when heating.

Heat milk

Pour the remaining 3 3/4 cups of whole milk into a medium saucepan. Warm over medium-low heat, stirring occasionally, until the milk is steaming and small bubbles appear around the edges (do not boil). This should take 4–6 minutes.

Combine and sweeten

Whisk the cocoa slurry into the warmed milk. Add the granulated sugar and the pinch of salt. Continue to gently whisk until the sugar dissolves and the mixture is smooth, about 1–2 minutes.

Add chocolate and flavor

Reduce heat to low. Chop or roughly shave the semi-sweet chocolate and add it to the saucepan. Stir continuously until the chocolate is fully melted and incorporated, creating a velvety texture. Remove from heat and stir in the vanilla extract.

Adjust and serve

Taste and adjust sweetness if needed (add up to 1 additional tablespoon sugar to taste). If the hot chocolate is too thick, thin with a splash of warm milk. Ladle into four warmed mugs (about 1 cup each).

Optional Garnish / Add-ins

Top each mug with a dollop of whipped cream if desired. Sprinkle with mini marshmallows, a dusting of cinnamon, and dark chocolate shavings for extra richness.

For a peppermint variation, add 1 drop of peppermint extract to the finished hot chocolate and stir quickly to combine.

Serve immediately while hot.