For the Oatmeal Cookies: Preheat oven to 350 degrees F. Line 2 large cookie sheets with parchment paper.
In a large bowl, or the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars until well combined, 3-5 minutes. Add egg and vanilla; mix until light and fluffy, about 2 minutes.
Add flour, baking soda, salt and cinnamon to butter mixture. Mix on low speed, just until flour disappears. Stir in quick oats until just combined.
Use a small cookie scoop to portion cookie dough onto prepared sheets. Space about 1 1/2 inches apart. Bake for 7 to 9 minutes, until edges start to turn golden brown. Remove cookies from oven and let sit for 3 minutes. Place cookies on a wire rack to cool completely before frosting.
For the Buttercream Frosting: In a large mixing bowl, beat all ingredients on medium speed until smooth and creamy, 2-3 minutes.
Once cookies have cooled completely, sandwich frosting between two cookies.
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