In a bowl, weigh the flour, baking soda, baking powder, salt, and cocoa powder. I whisk through the mixture to blend the dry ingredients and set aside. In a smaller bowl, weigh your soft dark brown sugar and set aside.
Add the softened butter and dark brown sugar (use soft brown sugar) to a stand or with a handheld mixer. You want to mix on a medium speed for 3 minutes. Next, add the vanilla extract and blend. Now, add the egg and egg yolk and mix again until incorporated.
Scrape down the sides and across the bottom of the bowl, and turn the mixer on at a low speed. Gradually add the flour mixture until incorporated. Avoid over-mixing the flour since you can always finish it with a spatula. Cover loosely and chill in the fridge for 30 minutes.
Preheat your oven to 350°F (175°C or 180°C). Have 2 cookie pans lined with parchment paper and set aside. I get a paper plate and spread the superfine white sugar around the plate to coat the cookie dough balls. I take a cookie scooper (medium) and put a scoop of cookie dough in my hand and roll it into a ball. Then, roll the ball in the superfine white sugar to coat. Next, add the ball to the cookie pan. I take 2 fingers and press down lightly on the top of each ball. Flattening them just a little helps the balls not to roll. The cookies spread as they bake, so I only did 9 balls to a pan.
Before I put the cookie pan into the oven, I put it back into the refrigerator for about 5 to 10 minutes for the butter to firm back up, then put them into the oven 1 pan at a time. Bake for 12-14 minutes, and keep an eye on them when they start to crack on the tops of the cookies; they are ready to come out of the oven. I leave them on the cookie pan for a minute and then carefully move them to a cooling rack. After they cooled, I took just a pinch of the white sugar and added it to the tops of the cookies; this, of course, is optional.