Make the Cookies:

1. Preheat and prepare: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats and set aside.
2. Mix dry ingredients: In a medium bowl, whisk together the oats, flour, baking soda, cinnamon, and salt until well combined. Set aside.
3. Cream butter and sugars: In a large bowl using an electric mixer or stand mixer fitted with the paddle attachment, beat the softened butter, brown sugar, and granulated sugar together on medium speed for 2-3 minutes until light and fluffy.
4. Add wet ingredients: Beat in the egg, molasses, and vanilla extract until fully incorporated and smooth, scraping down the sides of the bowl as needed.
5. Combine wet and dry: Add the dry ingredient mixture to the wet ingredients and mix on low speed just until combined. Don’t overmix—the dough should be thick and slightly sticky.
6. Shape cookies: Using a 1.5 tablespoon cookie scoop or spoon, drop rounded portions of dough onto the prepared baking sheets, spacing them about 2 inches apart. You should get about 36-40 cookies total. Gently flatten each mound slightly with your palm or the back of a spoon.
7. Bake: Bake for 9-11 minutes, until the edges are just set but the centers still look slightly underdone. The cookies will continue to cook on the hot pan after removing from the oven. Don’t overbake—you want soft, chewy cookies!
8. Cool completely: Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely before filling. This is crucial—filling warm cookies will cause the cream to melt.

Make the Cream Filling:

9. Beat butter and shortening: In a large bowl using an electric mixer, beat the softened butter and shortening together on medium speed for 1-2 minutes until smooth and creamy.
10. Add marshmallow fluff: Add the marshmallow fluff and beat on medium speed for another minute until well combined.
11. Add powdered sugar: With the mixer on low speed, gradually add the sifted powdered sugar, about ½ cup at a time, beating until smooth after each addition.
12. Add vanilla and cream: Beat in the vanilla extract. Add 1 tablespoon of heavy cream and beat until light and fluffy. If the filling is too thick to spread easily, add another tablespoon of cream. The filling should be smooth, fluffy, and easily spreadable but not runny.

Assemble the Cream Pies:

13. Fill cookies: Turn half of the cooled cookies upside down (flat side up). Using an offset spatula, butter knife, or piping bag, add about 1-2 tablespoons of cream filling to the center of each upturned cookie.
14. Create sandwiches: Top with another cookie (flat side down) and gently press together until the filling spreads to the edges. Don’t press too hard or the filling will squish out completely.
15. Set and serve: Let the assembled cream pies sit at room temperature for 15-20 minutes to allow the filling to set slightly before serving or storing.