In a large bowl, cream butter and sugars until light and fluffy. Beat in egg and vanilla. In a small bowl, combine banana and buttermilk. Combine the flour, salt, baking powder and baking soda; gradually add to creamed mixture alternately with banana mixture.
Drop by tablespoonfuls 2 in. apart onto parchment paper-lined baking sheets. Bake at 350° for 12-15 minutes or until set. Cool for 2 minutes before removing from pans to wire racks to cool completely.
For filling, in a large bowl, beat the cream cheese, peanut butter, and butter until fluffy. Beat in confectioners' sugar and vanilla until smooth. Spread filling on the bottoms of half of the cookies, about 1 tablespoon on each; top with remaining cookies. Dust with additional confectioners' sugar. Store in the refrigerator. Yield: 2 dozen.