Beat the butter, vegetable shortening, and confectioners’ sugar in a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer. Start on low speed, so the sugar doesn’t fly out of the bowl, and increase to medium-low and continue beating about a minute or two, until soft and creamy. Add the egg yolks and vanilla extract and continue beating until incorporated, about another minute. Reduce speed to low and add the flour, baking powder, and salt. Continue to mix until all the ingredients are incorporated, another minute. Wrap the dough in plastic wrap and refrigerate for at least 2 hours. (You can make the dough up to 2 days ahead.)

When you’re ready to bake, preheat the oven to 350°F with a rack set in the middle. Line two baking sheets with parchment paper.

Cut the dough in half leaving one half in the refrigerator. Gently flatten the dough and place between two sheets of parchment, wax paper, or plastic wrap. Using a rolling pin, roll out to about ½-inch thick (any thinner and the cookies will be too fragile to work with), aiming for a rectangular shape. Using a sharp knife or a bench scraper, cut the dough into rectangles of about ½-inch wide by 2-inches long. Using a dinner knife, carve two shallow lines down the length of each cookie.

Transfer the cookies to the baking sheets, placing them an inch apart and bake for 11-13 minutes, until lightly golden on the sides and bottoms. While the first batch bakes, you can roll out and shape the remaining dough, then bake. Cool on racks.

When the cookies are cool enough to handle, gently dredge them in a bowl of confectioners’ sugar. Store at room temperature in an airtight container.