Preheat oven to 275. Beat butter, peanut butter, brown sugar, egg, vanilla and salt together in a large bowl until smooth.
In a separate bowl combine the flour and baking soda. Slowly add dry mixture to the wet mixture while beating.
Roll 3 tbsp.size portions of the dough into a ball in your hands and press to 1/2 inch flat on a parchment lined baking sheet. Bake for 18-20 minutes. Be careful not to overcook, or the they will not be chewy.