Ingredients:
FOR THE COOKIES
1 cup unsalted butter, softened
0.75 cup firmly packed light brown sugar
0.5 cup granulated sugar
2 large eggs
1 tsp pure vanilla extract
1.5 cups all-purpose flour
1 tsp baking soda
1 tsp ground cinnamon
0.5 tsp salt
3 cups quick or old fashioned oatmeal

FOR THE CREAM FILLING
0.75 cup (1.5 sticks) cup unsalted butter, softened
2.5 cups confectioners’ sugar, sifted
1 tsp pure vanilla extract
1 tbs milk or half and half

Instructions:

Preheat oven to 350°F. Line cookie sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, cream together butter and sugars until light and fluffy, about 3 min. Add in eggs and vanilla and mix until combined.
In a medium bowl, whisk together the flour, baking soda, cinnamon and salt. Slowly add the flour mixture to the wet ingredients, mixing until just combined. Add in oats and mix until incorporated.
To make large cookies, use a large cookie scoop (about 3 tablespoons) to drop dough onto prepared cookie sheets. Bake in preheated oven for 13-15 min or until golden brown. Allow cookies to sit on cookie sheets for about 5 min before removing them to a wire rack to cool completely. Once cookies have cooled completely, prepare the cream filling.

Prepare the Cream Filling:
In the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium-high speed for about 5 min.
Turn the speed down to low and gradually add in the powdered sugar. Continue mixing on low speed until the powdered sugar is completely incorporated. Add in vanilla extract and milk and mix until combined.
Turn mixer back up to medium-high speed and beat the mixture for an additional 3-5 min.
To assemble the cookies, pipe or spread the cream filling on the flat side of half of the cookies and put the remaining cookies on top.