Peanut Butter Oatmeal Sandwich Cookies with Marshmallow Crème Filling

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. Combine oats, flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Add into peanut butter mixture and beat until just combined. 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. 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.