Line baking trays with parchment paper or silpat liners.

Wet ingredients: Add butter to a large mixing bowl or stand mixer and beat until smooth. Add sugars and mix together until creamed, light and fluffy, about 2-3 minutes. Add eggs and vanilla extract and mix until combined.

Dry Ingredients: In a separate bowl stir together oats, flour, baking soda, salt, and cinnamon.

Combine: Add to the wet mixture and mix until combined.

Chill: Cover dough and chill in the refrigerator for 30 minutes (or up to 2 days). Preheat oven to 350 degrees F.

Bake: Scoop dough into 1 Tablespoon-size balls, about 20 grams each (I use a 1-inch cookie scoop. Place on prepared baking sheets 2-inches apart. Bake for about 10 minutes, or until edges are lightly golden. Allow to cool on baking sheets for 10 minutes before moving to a cooling rack.

Cream filling: Mix butter with electric mixers until smooth. Add powdered sugar and 2 Tablespoons cream or milk, vanilla and mix for 2 minutes. Add a little more milk, if it’s too thick. Mix for 2 more minutes until very light and creamy.

Assemble: Scoop a small dollop of frosting onto the bottom of one cookie. Press a second cookie on top, to sandwich together.