How to Make It
Finely chop chocolate bars. In a medium bowl, whisk the flour, baking soda and salt. In a standing electric mixer fitted with a paddle, or using a hand mixer in a large bowl, beat the butter with the sugar at medium speed until light and fluffy, about 3 minutes. Beat in the egg yolk and vanilla, scraping the side and bottom of the bowl. Beat in the dry ingredients at medium-low speed, then fold in the chopped chocolate. Divide the dough in half. Pat each half into a round, wrap them in plastic and refrigerate for 15 minutes.
Preheat the oven to 350°. Position the racks in the upper and lower thirds of the oven. On a lightly floured surface, roll out each piece of dough 1/8-inch thick. Using a 2-inch biscuit cutter, stamp out rounds as close together as possible. Arrange the rounds on large baking sheets, about 1 inch apart. Gather the scraps and reroll, cutting out more cookies.
Bake the cookies in batches for about 20 minutes, until golden; shift the pans from top to bottom and front to back halfway through baking. Let the cookies cool on the sheets for 5 minutes, then transfer to racks to cool completely.