Prep the oven and pans: Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper.

Cream the fats and sugars: In a large bowl, beat the softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy, about 2 minutes. This step helps create that tender, chewy texture.

Add the egg and vanilla: Mix in the egg and vanilla until smooth. Scrape down the bowl so everything blends evenly.

Incorporate the sourdough starter: Stir in the sourdough discard until fully combined. The batter may look silky and slightly looser—that’s perfect.

Whisk dry ingredients: In a separate bowl, whisk the flour, baking soda, baking powder, and salt.

Combine wet and dry: Add the dry ingredients to the wet mixture. Mix on low just until no dry streaks remain. If using chips, fold them in now. Do not overmix.

Portion the dough: Scoop 1 1/2-tablespoon portions (a medium cookie scoop) and roll into balls. For a classic look, gently roll each ball in granulated sugar. Place on the prepared sheets, leaving about 2 inches between cookies.

Optional crisscross: Lightly press each dough ball with a fork in a crisscross pattern. Don’t flatten too much if you prefer a chewier, thicker cookie.

Bake: Bake for 9–12 minutes, until the edges look set and the centers are slightly underbaked. The cookies will puff and then settle as they cool.

Set and cool: Let the cookies rest on the baking sheet for 5 minutes, then transfer to a rack to cool. This helps lock in that soft center.