1Preheat the oven to 350F.
2In a large bowl, beat the butter and sugars until pale and light. Beat in the tahini, egg, egg yolk and vanilla. Stir in the flour, cocoa, baking powder, baking soda and salt, and then the chocolate. Cover and let sit for an hour, or (preferably) refrigerate overnight.
3When you're ready to bake them, preheat the oven to 350F. Spoon or scoop the dough onto a parchment-lined baking sheet, leaving an inch or two between each. Bake for 12-14 minutes, or until the cookies are set around the edges, but still soft in the middle. Transfer to a wire rack to cool. Makes about 2 dozen cookies.