Preheat oven to 325 degrees. Line two light-colored baking sheets with parchment paper and set aside
In a large bowl, cream butter and sugars together until light and fluffy, about 3-4 minutes. Stir in eggs and vanilla until well combined. Slowly incorporate remaining ingredients until the dough has just come together.
Using a medium cookie scoop, scoop into 36 cookies and place on prepared sheets. Bake 9-12 minutes or until edges are golden brown and the centers are soft but not gooey.
Cool 5 minutes on the hot baking sheet before transferring to cooling racks.
For the Marshmallow Filling:
In the bowl of a stand mixer, stir marshmallow fluff, butter, powdered sugar and vanilla together until it starts to come together. Give it some time to incorporate to a thick paste. Add milk sparingly 1 teaspoon at a time until you have a thick and creamy filling that will hold its shape. Transfer to a piping bag with a large round tip (I used the Wilton 1A).
Make the Oatmeal Cream Pies
Match cooled cookies together with similar sized partner cookies. Pipe about 2 tablespoons of marshmallow frosting onto the center of the bottom of a cookie. Press matching cookie onto the frosting to push filling towards the edges. Continue filling cookies until all 18 oatmeal cream pies are filled. Enjoy at room temperature.
Store leftover oatmeal cream pies in an airtight container at room temperature, dividing the layers with wax paper. If your cookies were baked a little crispy around the edges, add a piece of plain white sandwich bread to your airtight container, being sure the bread isn't touching the cookies directly. (I like to lay a piece of wax paper over the top of the cookies and then place the piece of bread on top, then seal with the lid. After 24 hours, the cookies should be completely soft.)
I use and prefer salted butter for all my baking. However, if you prefer unsalted butter, increase the salt to 1 teaspoon for the cookie dough and add 1/4 teaspoon of salt to the frosting.To make your Oatmeal Cream Pies with Cinnamon, add in 1 teaspoon ground cinnamon to your cookie dough with the dry ingredients in step 2.