Preheat the oven: Set your oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone mats.
Cream the butter, peanut butter, and sugars: In a large mixing bowl, the bowl of a stand mixer, beat the butter, peanut butter, brown sugar, and sugar together until light and fluffy. This should take about 2–3 minutes.
Add the wet ingredients: Beat in the egg and vanilla extract until fully incorporated. Then mix in the sourdough starter until smooth.
Combine the dry ingredients: In a separate bowl, whisk together the flour, baking soda, and salt.
Mix the dough: Gradually add the dry ingredients to the wet mixture, stirring until just combined. Avoid overmixing to keep the cookies tender.
Chill the dough: Cover the mixing bowl with plastic wrap and refrigerate the dough for 1–2 hours. This step helps the cookies hold their shape and enhances their flavor.
Shape the cookies: With a teaspoon or cookie scoop tablespoons of dough and roll them into balls. Place the cookie dough balls about 2 inches apart a prepared cookie sheet.
Create the classic crisscross pattern: Using the back of a fork, gently press down on each dough ball in one direction, then again in the opposite direction to create the signature peanut butter cookie look.
Bake: Place the cookies in the oven and bake for 10–12 minutes, or until the edges are lightly golden brown. The centers will still look slightly soft but will set as they cool.
Cool the cookies: Let the cookies rest on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Serve and enjoy: These delicious cookies are perfect as an afternoon snack or with a glass of milk!
