Preheat oven to 325 F. Line 2 baking sheets with parchment paper or silicone baking mats. Set aside.
Whisk flour, cocoa, baking powder, baking soda, and salt together in bowl. Please read the note about measuring flour.
Combine chocolate and butter in a microwave-safe bowl, and microwave at 50% power, stirring occasionally, until melted, about 2-3 minutes.
In a separate bowl, whisk brown sugar; eggs, and vanilla together until well-combined.
Whisk in the chocolate mixture until combined. Fold in flour mixture until no dry streaks remain. Let dough sit at room temperature for 10 minutes.
Fold in flour mixture until just incorporated.
Let dough sit at room temperature for 10 minutes.
Place granulated sugar and powdered sugar in separate shallow dishes.
Working with 1.5 tablespoons dough (a medium cookie scoop) at a time, drop scoop of dough into granulated sugar.
Roll in granulated sugar, and then roll in powdered sugar.
Place dough about 2 inches apart on prepared baking pan.
Bake cookies, 1 sheet at a time, until puffed and cracked and edges have begun to set but centers are still soft. The cookies will look raw between cracks and seem underdone, about 9-12 minutes.³
Cool the cookies on the baking sheet for about 10 minutes before removing to wire cooling rack.
Notes
Flour: To properly measure the flour, weigh or aerate it (stir it or sift it), then lightly spoon it into the measuring cup, and level. Packing the flour will give you a dry, crumbly cookie.
Cocoa powder: Either unsweetened baking cocoa or Dutch processed cocoa powder is ok.
Kosher salt: If you don't have kosher salt, you can use 1/4 teaspoon of regular (table) salt.
Brown sugar: Light or dark brown sugar will work.
Butter: I use salted butter in this recipe. If using unsalted butter, increase the salt by 1/4 teaspoon.
It's better that the cookies stay a little underbaked. That will help the cookies to stay soft.