Preheat the oven to 350 degrees F and line a rimmed baking sheet with parchment paper. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set to the side.
175 grams all-purpose flour, 4 grams baking soda, 1 gram baking powder, 3 grams salt
In a bowl of a stand mixer with a paddle attachment or with a handheld mixer, cream together the butter, peanut butter and sugars on medium speed until light and fluffy, about 4-5 minutes.
57 grams unsalted butter, softened, 190 grams creamy peanut butter, 50 grams granulated sugar, plus more for rolling, 220 grams brown sugar
On low, mix in the egg and egg yolk one at a time. Scrape down the sides when necessary.
1 large egg, 1 egg yolk
Add the sourdough discard and vanilla extract and mix on low until smooth.
100 grams sourdough discard, 100% hydration, 8 grams vanilla
Slowly add the dry ingredients into the wet ingredients until there are no more streaks of flour.
Scoop 45 gram portions of cookie dough, roll them until smooth and then roll them in some sugar. Repeat with the remaining cookie dough.
Place 8 cookie dough balls about 2 inches apart on the prepared baking sheet and bake for 10-12 minutes or until the edges are lightly golden brown and the centers have puffed up.
Keep the cookies on the baking sheet for 5 minutes and then cool completely on a wire rack. Repeat with the remaining dough balls.
As the cookies cool they will crack. Serve with milk and enjoy!