In a bowl weigh out the flour, baking soda, baking powder, and cocoa powder. I run a whisk through the mixture to blend the dry ingredients together and set aside.
In a smaller bowl weigh out your dark brown sugar and set aside.
In a stand mixer or handheld add the softened butter, dark brown sugar, salt, vanilla extract and blend till the mixture is smooth in texture. Add the egg and egg yolk and mix again till incorporated.
Stop the mixer and add half the flour mixture blend on a low-speed, mix for about 30 seconds. Stop the mixer again, scrape down the sides and add the remaining flour mixture and turn the blender on low again for another 30 seconds. You can mix with a spatula any small amount of the flour mixture but you don't want to over beat this or it will be really soft. If it is no worries just put in the freezer for about 10 minutes then transfer to the refrigerator for about an hour or two to finish firming up. This makes it easier to form into balls.
Turn your oven on to 350°. Have 2 cookie pans lined with parchment paper or a silicone mat and set aside. I get a paper plate and add the white sugar for coating. I take a cookie scoop (medium) and form into balls, then roll in the white sugar to coat and add to the cookie pan. I take 2 fingers and press down on the top of each ball lightly just enough to flatten them just a bit so they don't roll.
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 top they are ready to come out. I let them on the cookie pan for a minute and carefully move them to a cooling rack. After they cooled I took just a pinch of the white sugar and added to the top of the cookie, this is optional.