In a medium saucepan over medium heat, combine 3 cups whole milk and 1 cup heavy cream. Warm gently until small bubbles form at the edges but do not boil, stirring occasionally — about 5–7 minutes.

Make chocolate base

While milk warms, whisk together 3 tablespoons unsweetened cocoa powder and 3 tablespoons granulated sugar in a small bowl to remove lumps.

Incorporate cocoa

Once milk is steaming, whisk the cocoa-sugar mixture into the saucepan until fully dissolved and no lumps remain.

Melt chocolate

Reduce heat to low. Add 3 ounces chopped semi-sweet chocolate and a pinch of salt, stirring constantly until chocolate is fully melted and mixture is smooth, about 2–3 minutes.

Finish

Remove from heat and stir in 1 teaspoon vanilla extract. Taste and adjust sweetness if desired (add up to 1–2 teaspoons more sugar). Keep warm while preparing whipped cream and marshmallows.

Whipped Cream

Chill a medium mixing bowl and beaters (if available) in the refrigerator for 5–10 minutes. Pour 1 cup cold heavy cream into the bowl, add 2 teaspoons powdered sugar and 1 teaspoon vanilla extract.

Whip with a hand mixer or whisk until soft to medium peaks form — about 2–4 minutes with a mixer, longer by hand. Do not overwhip. Transfer to a small bowl and refrigerate until ready to serve.

Toasted Marshmallows & Assembly

Toast marshmallows

Skewer large marshmallows on long metal or heatproof skewers. Toast over an open flame (gas stove, grill, or kitchen torch) until golden and slightly charred, rotating to brown evenly — about 30–60 seconds per side. Alternatively, broil on a baking sheet on high for 30–60 seconds, watching carefully to avoid burning.

Assemble cups

Ladle hot chocolate into 4 heatproof mugs (about 1 cup per serving). Spoon or pipe a generous dollop of whipped cream on top of each mug.

Garnish

Top each with 1–2 toasted large marshmallows (place on a skewer across the mug or sit on the whipped cream). Sprinkle a few mini marshmallows if using, and dust with 1 teaspoon cocoa powder or grated chocolate divided among the mugs.

Serve immediately while hot. Offer extra marshmallows and whipped cream on the side.