Instructions

For the cookies:

1. Preheat oven to 350 degrees.
2. Beat butter and sugar until creamy, 3-4 minutes. Add the egg and beat on medium for about 1 minute. Add vanilla and beat until incorporated.
3. In a medium bowl, whisk together oats, flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Add this mixture to the butter mixture and beat on low until just incorporated. Stir in your mix-ins of choice.
4. Scoop dough by heaping tablespoons, rolling into a ball before placing them on a cookie sheet covered with parchment paper, spacing balls about 1 1/2 inches apart.
5. Bake for 10-13 minutes, until cookies are a little underdone. Let cool completely before sandwiching.

For the filling:

1. Beat butter, peanut butter, and powdered sugar on medium speed until smooth and creamy, 3-5 minutes. With mixer running, drizzle in the vanilla. Add milk 1 tablespoon at a time, until filling is smooth and will spread easily.
2. When the cookies have completely cooled, flip them over and spread one side with filling. Top with a similarly sized/shaped cookie.
3. Wrap individually and store in refrigerator. These cookies are best served slightly chilled.