In a large mixing bowl beat together the butter and brown sugar with an electric mixer until well combined, about 2 minutes.
Add the maple syrup and vanilla extract and beat to combine. Scrape down the sides of the bowl as needed.
Add the eggs one at a time, beating until combined.
Add the flour, cinnamon, salt and baking soda and beat until combined, scraping down the sides of the bowl as needed.
Add the oats and mix until well combined.
Cover the bowl with plastic wrap and refrigerate for at least 2 hours or overnight.
Heat the oven to 350º degrees. Line 2 baking sheets with parchment paper.
Portion the dough into 2-tablespoon scoops and arrange them on the prepared baking sheets, bake until golden, about 14 to 16 minutes.
Let cool for about 5 minutes and then transfer to a wire rack to cool completely.
In a large bowl, beat together the butter and powdered sugar until combined.
Add the maple extract and a dash of salt, and beat until combined.
Add the milk, one tablespoon at a time until desired consistency is met.
Spread 2 to 3 tablespoons of buttercream on ½ of the oatmeal cookies and then top with another cookie.
Store in an airtight container for 3 to 5 days. Enjoy!