Mix the flour, cocoa, salt and baking soda in a bowl and set aside. In another bowl, mix the cooled butter and sugars until they are combined. Add the egg, egg yolk, and vanilla and stir until mixed. Gradually add in dry ingredients and mix until a dough forms – it will look crumbly at first, but it will come together. I even used my hands to help at one point. Fold in chocolate chunks.