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

In a small bowl, combine the oats and milk. Let them sit for about 10 minutes to soak.

In a mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer until light and fluffy (about 3-4 minutes).

Beat in the eggs one at a time, then add vanilla extract, mixing until well combined.

In a separate bowl, mix together the flour, baking powder, cinnamon, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with the soaked oats, until just combined.

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

Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the tin for 5 minutes before transferring them to a cooling rack to cool completely.

For the cream filling, beat the softened butter with an electric mixer until creamy. Gradually add powdered sugar, vanilla extract, and milk, mixing until smooth and fluffy.

Once the cupcakes are completely cooled, use a small knife or a cupcake corer to create a small hole in the center of each cupcake.

Fill each hole with the cream filling, then replace the top part you removed.

Optional: top with additional cream filling or icing, if desired.