Using the right combination of sugars and shaping techniques guarantees thick and chewy cookies.
In medium bowl, whisk together flour, cocoa, baking powder, and salt.
Place chocolate chips in medium microwave-safe bowl. Heat in microwave at 50 percent power until melted, 2 to 3 minutes. Use oven mitts to remove bowl from microwave. Use rubber spatula to stir chocolate until completely melted and smooth.
In bowl of stand mixer (or large bowl if using handheld mixer), combine softened butter, brown sugar, and sugar. If using stand mixer, lock bowl into place and attach paddle to stand mixer. Start mixer on medium speed and beat until well combined, about 1½ minutes. Stop mixer and scrape down bowl.
Add egg, vanilla, and melted chocolate. Start mixer on low speed and mix until combined, about 2 minutes. Stop mixer and use rubber spatula to scrape down bowl.
Add flour mixture. Start mixer on low speed and mix until combined, about 1 minute. Stop mixer. Remove bowl from stand mixer, if using.
Scrape down bowl and stir in any remaining dry flour. Cover bowl with plastic wrap. Let dough sit at room temperature for 30 minutes.
Meanwhile, adjust oven rack to middle position and heat oven to 350 degrees. Line rimmed baking sheet with parchment paper.
When dough is ready, use your hands to roll dough into 12 balls (about 1 heaping tablespoon each). Place dough balls on parchment-lined baking sheet, leaving space between balls.
Place baking sheet in oven. Bake cookies until edges have just begun to set but centers are still very soft, 12 to 14 minutes.
Thick cookies are often chewy cookies—and we made recipe choices to create cookies that are nice and thick! First, we use more brown sugar than white sugar: Brown sugar contains more water, so it helps make cookies moister and chewier (see the How Sweet It Is experiment). Next, we let the dough rest for 30 minutes before shaping and baking the cookies. As the dough rests, the flour absorbs some of the water in the dough. The dough’s texture thickens up, which means that it won’t spread as much in the oven. We don’t press these cookies flat before baking, which helps prevent them from spreading and keeps them thicker. Finally, removing the cookies from the oven when they look almost done helps maintain their chewy texture.