Preheat the oven to 350°F (175°C).

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

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

In another bowl, combine the flour, baking soda, salt, and cinnamon.

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

Stir in the oats until evenly distributed.

Drop rounded tablespoons of dough onto a baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes or until the edges are golden brown.

Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar and beat until smooth.

Mix in the vanilla extract and milk until the desired consistency is reached.

Once the cookies are completely cooled, spread a generous amount of cream filling on the flat side of one cookie.

Top with another cookie, flat side down, to create a sandwich.

Repeat with the remaining cookies and filling.