Preheat oven to 350 degrees F. Line a sheet pan with parchment paper and set aside
In the bowl of a standmixer, fitted with a paddle attachment, add the unsalted butter, peanut butter, and sugar mix until fluffy and light, about 5 minutes. Scrape down the sides of the bowl with a nonstick spatula
In a small separate bowl, add the egg whites. Then using a fork, beat the egg whites with a fork until foamy and thin, then add to the butter and sugar. Mix on low until combined
Add in the flour, baking soda and salt. Gently mix to form a soft dough. You can use immediately. Alternatively, you can wrap the dough in cling wrap and refrigerate for one week (just let the dough come to room temp before you roll
Prepare the work surface by sprinkling flour on a clean space. This will ensure that the dough does not stick to the counter as you roll. Roll the dough out to about 1/8 of an inch thick. The thinner the cookie, the crispier, the thicker- the softer and chewier! Use your cookie cutter to cut (and stamp if you have a peanut cutter) the dough.
Transfer cut cookie dough pieces onto the prepared baking sheet. Making sure that you place them at least 1.5 inches apart. Bake for 11-15 minutes. Cookies will be firm and even in color.