Preheat oven to 350. In a large bowl with an electric mixer beat butter, sugar and PB until smooth. Mix in egg and vanilla until blended. Add flour, baking soda and salt and mix for 1 minute on med. speed. Spoon 1 inch balls of dough onto muffin tins that measure 3/4 inch deep. Bake 8-10 min or until lightly browned.
Take out cookies and press in a PB cup until only top shows, cool.