Preheat the oven to 350°F (180°C), or 340°F (170°C) for a fan-assisted oven. Line baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add the molasses, vanilla extract, and eggs. Beat until the mixture is smooth and creamy.
Add the flour, cinnamon, baking soda, and salt. Stir gently by hand a few times, then mix on low speed just until everything is combined.
Fold in the quick oats until evenly distributed throughout the cookie dough.
Scoop the dough into balls using about 1 to 1 1/2 tablespoons of dough per cookie. Arrange them approximately 2 inches apart on the prepared baking sheets.
Bake one sheet at a time in the center of the oven for 9–11 minutes, or until the tops appear set. Avoid overbaking if you want soft oatmeal cookies.
Leave the cookies on the baking sheet for about 10 minutes before transferring them to a wire rack. Cool completely before adding the filling.
To prepare the filling, beat the softened butter and cream cheese until smooth and creamy.
Gradually beat in 3 cups of powdered sugar, followed by the vanilla and salt. Add more powdered sugar as needed until the filling reaches a thick, spreadable consistency. If it becomes too thick, mix in a small amount of cream.
Pair cooled cookies of similar sizes. Spread or pipe the cream filling onto the flat side of one cookie and gently top with another.
Repeat until all the Homemade Oatmeal Cream Pies are assembled.