Preheat the oven to 350 F. Prepare two cookie sheets with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, or using a hand mixer, beat the butter, salt, vanilla and brown sugar until very light and fluffy, at least 5 minutes. Add the molasses and beat for another minute. Add the yolks and beat for another 2-3 minutes.

Sift in the cocoa, flour and baking soda. Beat until you have a smooth dough, about 1 minute. Using two tablespoons or an ice cream scoop, portion scoops of dough onto prepared cooking sheet. Roll the balls between your hands and dip the tops of them in the granulated sugar.

Bake for 9 minutes, they will puff up and crack in the oven. When they come out they’ll flatten. Allow to cool slightly before moving.