Prep your pan and oven. Line two baking sheets with parchment paper. Preheat the oven to 350°F (175°C).

Cream the butter and sugars. In a large bowl, beat the butter, brown sugar, and granulated sugar until light and fluffy, about 2–3 minutes. This step creates a soft, tender crumb.

Add eggs and vanilla. Beat in the eggs one at a time, then mix in the vanilla. Scrape the bowl so everything blends evenly.

Whisk dry ingredients. In a separate bowl, whisk the flour, cocoa powder, baking soda, baking powder, salt, and hot cocoa mix (if using) until no streaks remain.

Combine wet and dry. Add the dry ingredients to the butter mixture in two additions. Mix on low just until combined. The dough will be thick and slightly sticky.

Fold in chocolate and marshmallows. Stir in the chocolate chips or chunks. If using mini marshmallows in the dough, fold them in gently. Don’t overmix.

Portion the dough. Scoop into 1.5–2 tablespoon portions and place 2 inches apart on the prepared sheets. For extra neat edges, roll the scoops into smooth balls.

Bake. Bake for 9–11 minutes, until the edges are set and the centers look slightly soft and puffy. They’ll finish setting on the pan.

Cool. Let the cookies rest on the sheet for 5 minutes, then transfer to a rack to cool completely. Warm cookies are fragile, so handle with care.

Make the marshmallow drizzle. In a small bowl, whisk marshmallow creme, 1 tablespoon milk or cream, vanilla, and a pinch of salt until smooth and pourable. Add more milk a teaspoon at a time if needed.

Drizzle and finish. Once cookies are cool, drizzle the marshmallow mixture in thin zigzags. Add a light sprinkle of flaky salt or crushed candy canes while the drizzle is tacky. Let set for 15–20 minutes.