Step 1

Preheat oven to 300°. Line a baking pan with parchment paper or a silicone liner.

Step 2

Using an electric mixer on low speed, mix flour, cornstarch, sugar, cocoa powder, and salt. Add melted butter and continue mixing until the mixture starts to form small clumps.

Step 3

Spread crumbs on prepared pan, breaking them up if necessary. Bake for 20 minutes. Crumbs will be slightly moist to the touch but will dry and harden as they cool. Cool completely before using. Store in an airtight container for 1 week or refrigerate for 1 month.

Step 4

Whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

Step 5

Using an electric mixer on medium speed, beat butter, sugar, and corn syrup until combined. Add egg, vanilla, and melted chocolate, and beat until thoroughly combined.

Step 6

Reduce mixer speed to low. Gradually add flour mixture, mixing just until combined. Add chocolate crumbs and mix just until combined.

Step 7

Scoop dough by 3 tablespoons (a cookie scoop works great) onto a parchment-lined pan. Slightly flatten each cookie. Wrap the pan in plastic wrap. Refrigerate for at least 1 hour (or up to 1 week).

Step 8

Preheat oven to 375°.

Step 9

Place dough about 3 inches apart on baking sheets lined with parchment paper or silicone liners. Bake for 18 minutes. If cookies are still doughy in the center, bake for 1 more minute. Do not overbake.

Step 10

Cool cookies completely on pans.