In a medium mixing bowl, whisk together all ingredients except for the peanut butter.
Fold in the peanut butter until well combined. Transfer the dough to the fridge and let chill for ideally 1 hour, but at least 30 minutes.
Preheat the oven to 325 degrees Fahrenheit (not 350!)
Use a 1.5-2 inch medium cookie scoop and scoop the dough onto a baking sheet lined with parchment paper. Flatten the tops of the cookies with your fingertips slightly.
Sprinkle on additional sugar on top of the cookies if desired.
Bake for 11-15 minutes, depending on your oven. They should have crackly tops and a slightly golden bottom when they're done. They will harden quite a bit as they cool. During the final 5 minutes, remove the cookies from the oven and slam the baking sheet down (on your counter, stove top or other tough surface). This step is optional, but it will help the cookies spread properly and give them a nice crackly top. Even if they spread a lot, they'll still be chewy, don't worry!
Remove from the oven and let cool. I like to use a round cookie cutter as soon as they come out of oven and scoot it around the edges to create perfect circles. Once cool, sprinkle with salt flakes and enjoy!