HEAT oven to 350 degrees F. Line baking sheets with parchment paper, if desired. Beat brown sugar and butter in large bowl with electric mixer on medium speed until creamy. Add 1/2 cup peanut butter, egg and vanilla, beating until blended.
COMBINE flour, baking soda, salt, cinnamon and baking powder in medium bowl, stirring until well blended. Gradually add to creamed mixture, beating just until blended. Mix in oats.
DROP by 2 level teaspoonfuls, 2 inches apart, onto prepared baking sheets to make 48 mounds. Bake 9 to 11 minutes or until golden brown. Cool on baking sheet 3 minutes. Remove to wire rack to cool completely.
BEAT frosting and remaining 3/4 cup peanut butter in medium bowl with electric mixer on medium speed until blended. Spread in an even layer on the flat side of half the cookies. Place another cookie, flat side down, on top of filling mixture to make sandwich cookies.