In the bowl of a stand mixer fitted with the paddle attachment, cream together butter, vegetable shortening, and sugar on medium-low speed until soft and creamy, about 1-2 minutes.

Add Eggland's Best egg yolks and vanilla extract and mix until combined.

Reduce the speed to low, add in the flour, baking powder, and salt, and mix until incorporated.

Wrap the dough in plastic wrap and refrigerate for at least 2 hours or up to 2 days.

When you're ready to bake, position the racks in the upper and lower thirds of the oven and preheat to 350 degrees.

Unwrap the dough, divide it in half, and return the other half to the refrigerator.

Place one piece of dough between two lightly floured sheets of parchment paper or silicone baking mats.

With a rolling pin, roll dough to about ½-inch thick. The cookies will bake better if the dough is slightly more than ½-inch thick, rather than thinner.

Remove the top piece of parchment paper and, using a sharp knife or a bench scraper, cut the dough into rectangles about ½-inch wide by 2 inches long. Then, with a small spatula or the back of a fork or spoon, make two shallow lines going down the length of the cookie. Repeat with the second piece of refrigerated dough.

Slide the cookies on parchment paper onto a baking sheet spaced 1 inch apart and bake until lightly golden, 13-14 minutes.

Let cool on the baking sheet. After a few minutes, transfer to a bowl filled with confectioners' sugar and gently dip the cookies into the sugar to coat well. Transfer to a platter or store in an airtight container.