Chocolate Chip Shortbread Cookies
Cut the butter into pieces, place in a bowl, and freeze for 30 minutes. In a large bowl, combine the flour, powdered sugar and salt. Sprinkle the frozen butter pieces over the flour, and cut the butter into the flour with a pastry cutter until mixture is crumbly. You don't need to incorporate the butter too much - you should have some pieces that are slightly bigger than peas, and the mixture should still be very dry and floury.
At this point, stir the chocolate chips and nuts, if using them, into the flour butter mixture. In a small bowl, whisk together the egg yolks and vanilla. Add the egg vanilla mixture, a little at a time, to the flour mixture, stirring with a fork to moisten. Now, get your hands in there, working quickly, so that the heat of your hands does not warm the butter too much, and work the egg yolk into the dough for 1 minute.
Sprinkle the ice water, a teaspoon at a time, over the dough, working it in with your fingers as you would with pie dough, just until you are able to gather the dough together. You may need slightly more or less ice water.
Divide the dough into two equal balls. On sheets of waxed paper, shape the dough into 2 logs, about 2 inches in diameter, trying to keep them uniform in width the whole length. (Make sure the dough is pressed together firmly, so that you don't have any pockets of air trapped inside, which will cause cracks in the cookies.) Roll the logs up in the waxed paper, then roll those up in foil. Refrigerate for at least 4 hours, or overnight.
When the cookie dough is very cold and firm, unwrap, and slice into 1/4 inch thick slices. Freeze the cookie slices for 30 minutes.
While the cookie slices are chilling in the freezer, preheat the oven to 350 F. Bake the cookies on a Silpat baking mat (or on parchment paper) for 12 minutes. Since they will still appear very pale in color, and they do not rise or spread, you may have a hard time telling if they're done, but you'll see the bottom edges just starting to turn golden brown, which means they're ready. Cool the cookies for 2 minutes on the silpat, then transfer to a wire rack to cool completely.
Melt the bittersweet chocolate according to the directions on the package. Before dipping the cookies, place the wire cooling racks over a baking sheet to catch any drips. Dip the cooled cookies in the melted chocolate, so that just one half of the cookie is coated in chocolate, shake off the excess, and place on the wire rack. Sprinkle with the toffee bits before the chocolate sets. When you no longer have enough chocolate to dip the cookies in, scrape the remainder of the melted chocolate into a zip-lock bag, snip off the corner, and drizzle the remainder of the cookies with the chocolate. Place the cookies in the refrigerator for 30 minutes to completely set the chocolate. Once set, they can be stored at room temperature in an airtight container.
Yields 3 dozen cookies.
Shortbread cookie dough inspired by Dorie Greenspan's recipe for Sweet Tart Dough.