In a large bowl, use a hand mixer to beat together the butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs and vanilla, and beat until combined. Set aside.
In a separate medium bowl, whisk together the flour, cinnamon, baking soda, and salt. Add the dry ingredients to the butter mixture and mix on low speed just until no flour streaks remain. Pour in the oats and mix on low until combined. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes. Meanwhile, line two baking sheets with parchment and preheat the oven to 350° degrees.
After the dough chills, roll the dough into tablespoon-sized balls (ping pong-sized ball) and gently press it down, so it’s no longer rounded on the top. Place the dough on the baking sheets 2-inches apart.
Bake for 10 to 12 minutes or until the edges are very light brown. The centers will still be soft. Remove from the oven and allow the cookies to cool on the baking sheets before transferring to a wire rack to cool completely. While the cookies cool, make the filling.
Whoopie Pie Filling
In a medium bowl, use a hand mixer combine the powdered sugar, butter, milk, and vanilla. Mix until the filling is smooth and creamy. Add a teaspoon or two extra milk if necessary to thin out the filling. The filling should be spreadable but not too thin.
After the cookies cool, use a piping bag with a large, circular tip to place filling on the bottom of the cookie. Add a second cookie on top of the filling to form a sandwich.
Store the finished oatmeal cream pies in the refrigerator for up to 4 to 5 days.
The calories listed are an approximation based on the ingredients and a serving size of 1 whoopie pie. Actual calories will vary. The cookie sandwiches can be stored in a sealed container, in your fridge, for up to 4 days.