Instructions

Prepare the Hot Chocolate Crumb:

1. Heat the oven to 250°. In a medium bowl, whisk 1⁄2 cup of the hot chocolate mix with the flour, cornstarch, and salt. Add the butter and stir with spatula until the mixture forms small clusters. Spread the clusters onto a parchment paper-lined baking sheet and bake until dried and sandy, 20 minutes. Transfer the clusters to a rack and let cool completely.
2. Return the clusters to a bowl, break apart any that are larger than 1⁄2 inch in diameter, and then toss with the remaining 1⁄4 cup hot chocolate mix until evenly coated. Pour the milk chocolate over the crumbs and toss with the spatula until evenly coated, tossing every 5 minutes until the chocolate is cooled completely and no longer sticky. Transfer the crumbs to a container and refrigerate for up to 1 week.

Prepare the cookies:

1. Combine the butter, sugar, and glucose in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the egg, vanilla, and melted chocolate, and beat for 7 to 8 minutes.
2. Reduce the mixer speed to low and add the flour, hot chocolate mix, cocoa powder, baking powder, baking soda, and salt. Mix just until the dough comes together, no longer than 1 minute. (Do not walk away from the machine during this step, or you will risk overmixing the dough.) Scrape down the sides of the bowl with a spatula.
3. Still on low speed, add the hot chocolate crumbs and marshmallows and mix just until incorporated, about 30 seconds.
4. Using a 1.4oz ice cream scoop (or a 2.5 tablespoons), portion out the dough onto a parchment-lined sheet pan. Pat the tops of the cookie dough domes flat. Wrap the sheet pan tightly in plastic wrap and refrigerate for at least 1 hour, or up to 1 week. Do not bake your cookies from room temperature–they will not bake properly.
5. Heat the oven to 350°F.
6. Arrange the chilled dough a minimum of 4 inches apart on parchment- or Silpat-lined sheet pans. Bake for 14 minutes. The cookies will puff, crackle, and spread. It’s tough (kind of impossible) to gauge if a cookie that is this dark with chocolate is done. If after 14 minutes, the cookies still seem doughy in the center, give them another 1 minute in the oven, but not more.
7. Cool the cookies completely on the sheet pans before transferring to a plate or an airtight container for storage. At room temp, the cookies will keep fresh for 5 days; in the freezer, they will keep for 1 month.