(1 sandwich cookie)
Preheat oven to 350°F. For the Brownie Cookies, mix flour, baking powder and salt in medium bowl. Mix granulated sugar and cocoa powder in medium bowl. Set aside. Microwave butter in large microwavable bowl on HIGH 2 minutes or until melted. Stir in cocoa-sugar mixture. Add eggs, 1 at a time, stirring until well blended after each addition. Stir in vanilla. Gradually stir in flour mixture until well mixed.
Drop by scant tablespoons about 2 inches apart onto parchment paper-lined baking sheets.
Bake 8 to 10 minutes or until cookies are set. Cool on baking sheets 1 minute. Remove to wire racks; cool completely.
For the Peanut Butter Filling, mix all ingredients in large bowl with electric mixer on medium speed until smooth. If filling is too thick to spread, gradually beat in additional 1 tablespoon milk. To assemble sandwich cookies, place about 1 tablespoon Filling on the flat side of 1 cookie. Top with a second cookie, pressing gently to spread the filling. Repeat with remaining cookies.