• Preheat the oven to 350°F and line your cookie sheets with parchment paper. In the bowl of a stand mixer fitted with the paddle attachment, or using an electric mixer in a large bowl, cream together the butter and sugars until light and fluffy, about 3 minutes. Add the eggs and vanilla extract, mixing until well combined.
• In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt. Slowly add the dry ingredients to the wet ingredients, mixing until just combined. Stir in the oats and mix until evenly incorporated.
• For large cookies, use a large cookie scoop (approximately 3 tablespoons) to drop dough onto the prepared cookie sheets. Bake in the preheated oven for 13-15 minutes, or until golden brown. Let the cookies sit on the sheets for about 5 minutes before transferring them to a wire rack to cool completely. For smaller cookies, use a medium cookie scoop (about 1.5 tablespoons) per cookie and bake for 10-12 minutes. Once the cookies are completely cooled, prepare the cream filling.

To prepare the cream filling:

• In the bowl of a stand mixer with the paddle attachment, beat the butter on medium-high speed for about 5 minutes. This will lighten the color of the butter and make the buttercream extra fluffy. Lower the speed to low and gradually add the powdered sugar, continuing to mix on low until fully incorporated. Add the vanilla extract and milk, mixing until combined.
• Increase the speed back to medium-high and beat for an additional 3-5 minutes. If necessary, add more cream, 1 teaspoon at a time, until you reach the desired consistency.
• To assemble the cookies, pipe or spread the cream filling on the flat side of half of the cookies, then sandwich with the remaining cookies.