Preheat your oven to 350°F (175°C) and line a cupcake tin with cupcake liners.
In a small bowl, combine rolled oats and milk. Allow it to sit for about 10 minutes until the oats soften.
In another bowl, whisk together the flour, baking soda, cinnamon, and salt.
In a large mixing bowl, beat together the softened butter and brown sugar until creamy. Add in the eggs one at a time, followed by the vanilla extract and the oat mixture.
Gradually mix in the dry ingredients until just combined.
Divide the batter evenly among the cupcake liners, filling each about ⅔ full. Bake for 15 to 18 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cupcakes to cool in the tin for 5 minutes, then transfer to a cooling rack to cool completely.
While the cupcakes are cooling, prepare the cream filling by mixing together powdered sugar, softened butter, vanilla extract, and milk until smooth.
Once cupcakes are completely cool, pipe or spoon the cream filling onto the top of each cupcake.
For the frosting, beat the softened butter until creamy, then gradually mix in powdered sugar. Add milk and vanilla extract and beat until fluffy.
Pipe the frosting over the cream filling to finish the cupcakes.