Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a medium bowl, combine flour, oats, baking powder, baking soda, salt, and cinnamon. Whisk together and set aside.

In a large bowl, use an electric mixer to cream the softened butter and brown sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture, mixing until well combined.

Gradually add the dry ingredients to the wet mixture, alternating with milk. Begin and end with the dry ingredients, mixing just until combined.

Scoop the batter into the prepared cupcake liners, filling each about ⅔ full.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 5 minutes before transferring to a cooling rack.

While the cupcakes cool, prepare the cream filling. In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar and marshmallow fluff, mixing until smooth. Stir in vanilla extract.

Once the cupcakes have cooled, use a piping bag or a knife to add the cream filling on top of half of the cupcakes. Sandwich them with the other half.

Serve and enjoy your Oatmeal Cream Pie Cupcakes!