Preheat Oven:
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Make Cookie Dough:
In a large bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy. Beat in eggs and vanilla extract.
Combine Dry Ingredients:
In a separate bowl, whisk together flour, baking soda, salt, and cinnamon. Gradually mix into the butter mixture. Stir in oats until evenly combined.
Shape Cookies:
Scoop tablespoon-sized portions of dough and place on prepared baking sheets, spacing about 2 inches apart. Flatten slightly with the back of a spoon.
Bake Cookies:
Bake for 10–12 minutes, until edges are lightly golden. Allow cookies to cool on baking sheets for 5 minutes, then transfer to wire racks to cool completely.
Make Cream Filling:
In a medium bowl, beat together butter and shortening until smooth. Gradually add powdered sugar, beating until creamy. Mix in vanilla extract and milk until fluffy. Adjust milk for desired consistency.
Assemble Oatmeal Cream Pies:
Spread or pipe cream filling onto the flat side of one cookie, then sandwich with another cookie. Press gently to adhere.
Serve:
Enjoy immediately or chill for firmer filling.