Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a medium bowl, combine the rolled oats, flour, baking soda, cinnamon, and salt. Mix well and set aside.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. If using, fold in the mini chocolate chips.
Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are lightly golden. Remove from the oven and let cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.
While the cookies are cooling, prepare the cream filling. In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar and mix until smooth. Stir in the vanilla extract and add milk as needed to achieve a spreadable consistency.
Once the cookies are completely cool, spread a generous amount of cream filling on the flat side of one cookie and top it with another cookie to create a sandwich. Repeat with the remaining cookies.