Preparation

• 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.
• 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.
• Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper.
• Remove cookie dough from the refrigerator. Scoop and roll dough, a heaping 1 Tablespoon 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 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.
• Melt the chocolate: 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.
• Chocolate sets at room temperature in 30-60 minutes. Once chocolate has set, you can stack, store, and gift the cookies.
• Cover leftover cookies tightly and store at room temperature for up to 1 week.