Chewy Oatmeal Sandwich Cookie

Name: Chewy Oatmeal Sandwich Cookie
Ingredients: For the cookies:; 2 1/12 cups old-fashioned rolled oats; 1 3/4 cups all-purpose flour, spooned and leveled; 1 teaspoon ground cinnamon; 3/4 teaspoon baking soda; 1/2 teaspoon fine salt; 2 sticks (1 cup) unsalted butter, at room temperature; 1 1/2 cups packed light brown sugar; 2 large eggs; 1 teaspoon pure vanilla extract; For the filling:; 1/2 cup (1 stick) unsalted butter, at room temperature; 1 1/4 cups marshmallow fluff; 1/4 teaspoon fine salt