Pre-heat your oven to 350°F.
Cream together the butter, peanut butter, and sugars for two to three in a stand mixer or with an electric mixer.
Whisk together the flour, baking soda, cornstarch, and salt in a separate bowl.
Add the dry ingredients to the wet gradually (about a quarter of it at a time), mixing just until combined between additions.
Cover the dough with plastic wrap and chill in the fridge from 30 minutes to overnight.
Scoop the dough into 1 1/2 tablespoon portions and roll into balls with the palms of your hands. For the most consistent cookies, weigh your dough and divide by 18. Then weigh the individual balls.
Roll the balls in granulated sugar before placing them onto a parchment-lined baking sheet and pressing them with the tongs of a fork in a criss-cross pattern. Keep about 2.5 inches between the cookies, as they will spread some.
Bake at 350 for 9-10 minutes. The sides should be barely golden brown and the center should look under baked. The longer they cook, the more dry the will be.
Let cool completely before enjoying.