Preheat oven to 350 degrees F and line 2 baking sheets with parchment paper.

Whisk the cocoa powder, flour, salt and baking powder in a small mixing bowl until thoroughly combined. Set aside.

Beat the butter and sugar in the bowl of an electric stand mixer fitted with the paddle attachment on medium-high power until light and fluffy (about 4-5 minutes), scraping down the sides of the bowl once or twice.

Add the eggs (one at a time), mixing after each until thoroughly combined, then add the vanilla and mix in.

Scrape down the sides of the bowl, then add the dry ingredients and mix on the lowest speed (if you don't, you'll have cocoa powder everywhere), mixing just until everything is combined (avoid over-mixing!).

Scoop out using a 1" cookie scoop or tablespoon, and round them into balls, then place on the parchment-lined baking sheets. Sprinkle a little granulated sugar on top of each cookie, if desired.

Bake in preheated oven for 9-10 minutes. It is important not to overbake these! Keep in mind they will continue to cook on the tray once taken out of the oven. When they're ready to take out of the oven, the cookies will still be smooshy but the edges will have just started to crack a little bit.

Allow to cool on the tray for 3-4 minutes, then transfer to wire cooling racks and allow to cool completely.

These can be stored in an air-tight container in the refrigerator for up to a week, or in an air-tight container at room temperature for a few days.