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

In a small bowl, combine the old-fashioned oats and milk. Let it sit for about 10 minutes to allow the oats to soften.

In a larger mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer or whisk until light and fluffy.

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

In another bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Then fold in the oat and milk mixture until evenly incorporated.

Divide the batter evenly among the cupcake liners, filling each about ⅔ full. Bake in the preheated oven for 16-18 minutes or until a toothpick inserted in the center comes out clean.

Once done, remove the cupcakes from the oven and let them cool in the tin for about 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 the softened butter until creamy. Gradually add the powdered sugar and mix until smooth.

Add the vanilla extract and milk, mixing until you achieve a creamy consistency.

Once the cupcakes are completely cooled, use a piping bag (or a knife) to fill each cupcake with the cream filling from the top or by cutting a small hole in the center and inserting the filling.

Optionally, sprinkle additional powdered sugar on top of the filled cupcakes before serving.