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

In a bowl, combine rolled oats, flour, baking powder, baking soda, cinnamon, and salt. Mix well and set aside.

In another mixing bowl, cream together the butter, brown sugar, and granulated sugar using an electric mixer until light and fluffy.

Beat in the eggs one at a time, followed by the vanilla extract, mixing until fully incorporated.

Gradually add the dry ingredients to the wet mixture, alternating with milk, and mix until just combined. Do not overmix.

Divide the batter evenly among the cupcake liners, filling them about two-thirds full.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and let the cupcakes cool in the tin for 5 minutes before transferring them to a cooling rack to cool completely.

While the cupcakes cool, prepare the cream filling. In a mixing bowl, beat together the softened butter, powdered sugar, vanilla extract, and marshmallow fluff (if using) until smooth and creamy.

Once the cupcakes are completely cool, use a piping bag or a simple spoon to fill the center of each cupcake with the cream filling.

Optionally, you can top each filled cupcake with a dollop of additional cream filling for decoration.