Preheat oven to 350°. Line a baking sheet with parchment paper and set aside.
In a medium bowl, mix together flour, baking soda, salt, cinnamon, and oats. Set aside.
In a large bowl, beat the butter and sugars together until light and creamy. Add in the eggs, vanilla, and molasses. Mix until combined, scraping the sides of the bowl when needed.
Slowly add the dry ingredients to the wet ingredients. Your batter should be very thick and you might need to mix it by hand after it initally combines. Use a large cookie scoop (or two tablespoons worth) to scoop the dough, roll into a ball, and drop them onto your baking sheet. I put 6 cookies on at a time since they are larger and tend to spread out more.
Bake cookies for 9-10 minutes, until lightly golden brown around the edges. You don't want to overbake these because we want them to be soft. Allow them to cool on the baking sheet for about 5 minutes before moving them to a cooling rack.
For the filling: Using an electric mixer, beat the butter on medium speed until smooth, about 2 minutes. Mix in the powdered sugar, vanilla, and heavy cream until combined, and then beat on high speed for about 3-4 minutes. If the filling seems way too thick, add a couple more teaspoons of heavy cream. If it's too thin, add a tbsp of powdered sugar.
You can use either a piping bag/Ziploc bag to pipe the frosting, or just spread 1½ tbsp of creme onto the bottom half of the cookies; top with remaining cookies to make a sandwich.