In a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment, beat unsalted butter, dark brown sugar, and white sugar together until fluffy. Scrape down the sides of the bowl with a spatula.
On medium speed add eggs, vanilla, and molasses. Mix until combined.
In a medium bowl, whisk together the flour mixture—flour, baking soda, baking powder, salt, and cinnamon. Add dry ingredients into the creamed butter mixture on low speed until just combined. Stir in oats to form an oat mixture.
Preheat oven to 350°F (175°C). Line baking sheets or cookie sheets with parchment paper. Scoop dough with a cookie scoop (about a 2-tablespoon scoop, or a heaping tablespoon) onto sheets, spacing 2 inches apart.
Bake for 9–11 minutes, until edges are golden brown but centers are soft. Cool on a wire rack.
In a medium bowl, beat the butter until creamy. Add marshmallow creme, powdered sugar, salt, and vanilla. Using a whisk attachment, beat on medium-high speed until smooth and fluffy. If needed add 1-2 teaspoons milk for spreading consistency. This is your sweet cream filling—perfect for sandwiching!
Turn cookies over with the bottom side facing up. Place a heaping tablespoon or pipe a layer of sweet cream filling with a piping bag in the center of the cookie. Press another cookie on top to create cookie sandwiches.
(Optional) Chill for 1-2 hours to help the filling set.