Oatmeal Cookies:
Preheat the oven to 350F. Line cookie sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and brown sugar for 2 to 3 minutes. Add in egg and vanilla - mix until combined.
Add flour, baking soda, salt, and cinnamon. Beat until combined.
Add in the oats and mix until incorporated.
Use a 1-tablespoon cookie scoop to measure out the dough. Drop them evenly on the prepared cookie sheets. Bake for 10 minutes.
Let the cookies cool completely before adding the cream filling.
Cream Filling:
Add butter, powdered sugar, vanilla, milk, and salt to a bowl of a stand or hand mixer and beat until thick and creamy, about 5 minutes.
Assemble Cookies:
Using a butter knife or spatula, spread the cream filling on the flat side of half of the cookies. Place the remaining cookies on top to make sandwiches and lightly press the cookies together.