In your mixing bowl, add the softened butter, peanut butter, white sugar, and brown sugar. Cream together on high with beaters or your paddle attachment for 2-3 minutes until the mixture is smooth and fluffy.
1/2 (120g) cup creamy peanut butter, 1/3 (76g) cup unsalted butter, 1/2 (100g) cup brown sugar, packed, 1/2 (100g) cup granulated white sugar
Add the sourdough discard, egg, and vanilla to the bowl. Whip in the ingredients for another minute until everything is combined.
1/3 (80g) cup sourdough discard , 1 large egg, 1 tsp vanilla extract
In a separate bowl, add the flour, cornstarch, baking soda, and sea salt. Whisk the dry ingredients together.
1 1/4 (155g) cups all-purpose flour, 1/2 Tbsp cornstarch, 1/2 tsp baking soda, 3/4 tsp salt
Add the dry ingredients to the wet, and gently mix them in until the dough is just combined and no flour streaks remain.
Cover the bowl with plastic, and transfer to the refrigerator to chill for at least an hour.
Overnight Fermentation Option: For optimal fermentation, consider chilling the dough overnight. Simply leave the dough in the refrigerator overnight and shape the next day.