In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium high speed until fluffy and light in color, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and vanilla extract, and then beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, cocoa powder, hot cocoa mix, baking soda, and salt together until combined. Add the dry ingredients to the bowl with the wet ingredients. Beat on low until combined. The cookie dough will be quite thick. Finally, beat in the milk. The cookie dough will be thick and sticky.

Cover dough tightly and chill in the refrigerator for at least 2 hours and up to 3 days. Chilling is imperative for this sticky cookie dough.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Remove cookie dough from the refrigerator. Scoop and roll dough, a heaping 1 Tablespoon (about 25-26g) each, into balls. Arrange 2–3 inches apart on the baking sheets.

Bake the cookies for just 10 minutes. Remove the cookies from the oven and lightly press a marshmallow half into the tops of each cookie, cut side down. Return them to the oven and bake for 2 more minutes. Remove from the oven and, using the back of a spoon, gently press down on the marshmallow to slightly flatten it out.

Cool cookies on the baking sheet for 10 minutes, and then transfer cookies to a wire rack to cool completely.

You can melt the chocolate in a double boiler or the microwave. If using the microwave, place the chopped chocolate in a medium heat-proof bowl. Microwave for 20-second increments, stirring after each increment until completely melted and smooth. Spoon melted chocolate over each cooled marshmallow-topped cookie. (When covering with melted chocolate, I usually put the cookies back on a baking sheet, or you can just keep them on the wire rack.) Chocolate sets at room temperature in 30-60 minutes. Once chocolate has set, you can stack, store, transport, and gift the cookies.

Cover leftover cookies tightly and store at room temperature for up to 1 week.