Peppermint Whoopie Pies

For Cookies: Preheat oven to 350°F (175°C). In medium bowl, combine flour, cocoa powder, baking powder, baking soda and salt; set aside. In large bowl, cream butter and brown sugar with electric mixer until light and fluffy. Add egg and extracts; mix well. Slowly add flour mixture and buttermilk, alternating, beginning and ending with flour mixture. Cover dough and chill at least 30 minutes. Drop dough by rounded tablespoons onto parchment-lined baking sheet, placing cookies 2 inches apart. Bake cookies 9 to 12 minutes, or until firm. Cool cookies on pan 3 to 5 minutes or until set, then transfer to wire rack to cool completely. For Filling: In large bowl, cream together butter, fluff, and vanilla extract until light and fluffy. Add powdered sugar slowly; mix well. Spread flat side of half the cookies with heaping tablespoon of filling; top with second cookie to create sandwich. Roll edges in crushed peppermint candies; chill cookies to set filling. Store unfilled cookies and filling in airtight containers in refrigerator for up to 1 week, and filled cookies for up to 3 days. Bring cookies to room temperature before serving.