• In large bowl, cream together butter, brown sugar, and white sugar until light and fluffy. Beat in vanilla extract and peanut butter. Beat in egg until well blended.
  1 stick (½ cup) unsalted butter, ¾ cup light brown sugar, ¼ cup granulated sugar, 1 tablespoon vanilla extract, ½ cup creamy peanut butter, 1 large egg
• Combine oats, flour, baking powder, baking soda, cinnamon, and salt. Add into peanut butter mixture and beat until just combined.
  ¾ cup quick oats, 1 cup all-purpose flour, ½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon cinnamon, ½ teaspoon salt
• Chill cookie dough, covered, at least 2 hours and up to 1 week.
• Preheat oven to 350 degrees F. Form rounded tablespoons of dough into balls and arrange about 2 inches apart on baking sheets lined with parchment paper.
• Bake cookies in batches in middle of oven for 10 to 11 minutes, or until just pale golden. Cool cookies on baking sheet 5 minutes and transfer to racks to cool completely.
• While cookies are cooling, prepare marshmallow buttercream filling.

For the filling

• Combine all ingredients in large bowl and mix on high speed with an electric mixer for 1 to 2 minutes or until light and fluffy.
  7 oz. jar Marshmallow Fluff, ½ cup (1 stick) unsalted butter, ⅓ cup powdered sugar, 1 tablespoons vanilla extract, ⅛ teaspoon salt
• Match cookies in pairs based on size. Spread 1 tablespoon of buttercream onto one cookie and sandwich with other cookie in the pair. Repeat until all cookies are sandwiched.