Step 1

Adjust oven rack to lower-middle position and preheat oven to 350°F (180°C). Sift flour and cocoa powder together in a medium bowl. Chop candy into roughly 1/4-inch pieces.

Step 2

Combine butter, white sugar, brown sugar, salt, baking soda, baking powder, and vanilla in the bowl of a stand mixer fitted with a paddle attachment. Mix on low to moisten, then increase to medium and beat until soft and light, about 5 minutes. With mixer running, add egg and continue beating until smooth. Scrape bowl and beater with a flexible spatula, then resume mixing on low. Sprinkle in flour/cocoa; once it is incorporated, add chopped candy and continue mixing until well combined.

Step 3

Divide dough into roughly 32 portions of 1 1/2 ounces each. (If you like, some dough can be frozen for up to 3 months in heavy-duty zip-top bags, then brought to room temperature for baking.) Arrange portioned dough on a parchment-lined aluminum half-sheet pan, shaping each piece into a craggy, cookie-shaped blob. If you like, top cookies with a few extra candy pieces. Bake until puffed and firm around the edges, though steamy and soft in the middle, about 15 minutes. Cool to room temperature directly on sheet pan; cookies will be extraordinarily fragile while warm. Store in an airtight container at room temperature for up to 3 days.