To make 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, 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 chocolate crumbs and mix just until incorporated, about 30 seconds.
4. Using a 2 3/4-ounce ice cream scoop (or a 13-cup measure), 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 375 degrees F.
6. Arrange the chilled dough a minimum of 4 inches apart on parchment- or Silpat-lined sheet pans. Bake for 18 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 18 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.
To make the chocolate crumb:
1. Heat the oven to 300 degrees F.
2. Combine the flour, cornstarch, sugar, cocoa powder, and salt in the bowl of a stand mixer fitted with the paddle attachment and paddle on low speed until mixed.
3. Add the butter and paddle on low speed until the mixture starts to come together in small clusters.
4. Spread the clusters on a parchment- or Silpat-lined sheet pan. Bake for 20 minutes, breaking them up occasionally. The crumbs should still be slightly moist to the touch at that point; they will dry and harden as they cool.
5. Let the crumbs cool completely before using in a recipe or eating. Stored in an airtight container, they will keep fresh for 1 week at room temperature or 1 month in the fridge or freezer.
Recipe from Momofuku Milk Bar (c) 2011, by Christina Tosi. Reprinted with permission by Crown Publishing Group.