In a bowl, combine the flour, cocoa powder, salt, and baking soda. Stir to blend and set aside.
In a mixing bowl with an electric mixer, beat the melted butter with brown sugar and granulated sugar until smooth. Cool slightly and then beat in egg and egg yolk. Beat in vanilla.
Slowly beat the dry ingredients into the butter and sugar mixture until well blended. Fold in chocolate chips.
Chill the dough for at least 1 hour, or until firm.
Heat oven to 350 F.
Using a cookie scoop or tablespoon, scoop mounds of dough onto a silicone-lined or greased baking sheet, leaving about 2 inches between cookies.
Bake for 12 to 15 minutes, until set.
Cool the cookies on the baking sheet on a rack for a few minutes and then remove them to rack to cool completely.