Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
Add butter, brown sugar, and ¼ cup (50 grams) granulated sugar to a large bowl or stand mixer fitted with a paddle attachment. Beat together on medium speed for 2 minutes using a handheld electric or stand mixer until the mixture is light and fluffy. Add in egg and vanilla extract and beat on medium speed until combined.
Scrape down the side of the bowl, then add the dry ingredients to the wet ingredients in two batches. Mix on medium-low speed until fully incorporated. The dough will be slightly sticky. Wrap cookie dough in parchment paper or plastic wrap and refrigerate for at least 3 hours or up to overnight until the dough has firmed up.
Meanwhile, pour 3 tablespoons (40 grams) of granulated sugar on a plate and set aside.
Heat oven to 350°F (175°C).
Remove dough from the refrigerator and form into 2 tablespoon balls. Roll the outside of the dough balls in the granulated sugar. Place dough 1-2 inches (2.5 - 5 centimeters) on a large baking sheet lined with a silicone baking mat or parchment paper. Use your thumb or the end of a spatula to make an indent in each cookie. If the dough begins to crack on the outside, simply use your hands to push it back together.
Bake on the middle rack for 12-14 minutes, or until cookie edges are set. Remove from the oven and let stand for 5 minutes. If any of the indents have puffed up, use a small spoon to push the indent down. Transfer to a cooling rack to fully cool.