In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth.

Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Stir in the rolled oats until evenly distributed.

Drop by rounded tablespoons onto ungreased baking sheets and put them in the freezer for 30 minutes.

Bake for 15 minutes, or until the edges are golden brown then let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

In a large bowl, cream the softened butter until smooth and fluffy.

Gradually add the powdered sugar, beating until well combined.

Stir in the vanilla extract. Add milk, one tablespoon at a time, until the filling reaches a smooth and spreadable consistency.

Match up cookies of similar size then spread a generous amount of filling onto the flat side of one cookie.

Top with another cookie, flat side down, to create a sandwich then repeat with the remaining cookies and filling.

Serve immediately, or wrap individually and store in the refrigerator for later.