Directions

1. Preheat oven to 350° F.
2. In the bowl of a stand mixer (or by hand), cream the butter and the sugars. Add the egg yolks and the vanilla and mix well.
3. Combine the flour with the baking powder and the salt in a separate bowl. Add the dry ingredients to the wet ingredients and mix until well blended. [Editor's note: We added an additional egg yolk when we noticed the dough was crumbly.]
4. If you want to make rolled/cut-out cookies, you should wrap the dough in parchment or plastic and refrigerate until it is thoroughly chilled. You could also make a "log" out of the dough, freeze it, and then make "slice and bake" cookies. The third option (and what I usually do) is to shape the dough into balls, which you can flatten a bit with a metal spatula or the bottom of a glass (use a glass with a pattern and you'll end up with patterned cookies). Don't flatten them too much, because a bit of thickness helps them keep their shape/ensures they stay chewy, and place on parchment or silpat lined cookie sheets.
5. Bake for 12 to 15 minutes, or until slightly brown on the bottoms and around the edges. In my oven, this takes 14 minutes.