In a large mixing bowl, cream the butter and shortening until whipped and creamy. Add the peanut butter and sugars and beat until light and fluffy. Add eggs, one at a time, mixing well after each addition. Mix in vanilla. Combine the flour, baking soda, and salt in a separate bowl. Gradually add to the creamed mixture.
Cover and chill for 1-3 hours.
Shape into 1 inch balls. Place 2 inches apart on ungreased baking sheets. Flatten and make criss-crosses with a fork. Bake at 375 degrees for 7-8 minutes or until golden. Remove to a wire rack and cool completely.
For the filling: Combine all ingredients and mix until smooth. Spread on the bottom of half the cookies. Top with the remaining cookies.