Mix starter, peanut butter, butter, and brown sugar. Add eggs and vanilla extract, and beat well.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Add flour mixture to starter mixture, adding additional flour if needed.
Chill dough for up to 30 minutes, or until it becomes easier to handle.
Roll dough into small balls. Roll balls in sugar, then gently flatten with the bottom of a mason jar.
Using a fork, gently press down with the back end so the fork tines leave an impression on the top of the dough. Turn your fork 90 degrees and do it again to create a criss-cross pattern.
Bake for 10 minutes or until cookies are light brown and set.