Preheat the oven to 350°F (175°C) and line a cupcake pan with cupcake liners.
In a small bowl, combine the rolled oats and milk; let it sit to soak for about 15 minutes.
In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition, and then stir in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, alternating with the oat mixture; mix until just combined.
Fill each cupcake liner about ⅔ full with the batter. Bake in the preheated oven for 15-18 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.
For the cream filling, beat the softened butter in a mixing bowl until creamy. Gradually add the powdered sugar and mix until well combined.
Add the vanilla extract and milk, mixing until smooth and fluffy.
Once the cupcakes are cooled, use a piping bag or a knife to fill the center of each cupcake with the cream filling, or spread it on top as desired.