Peanut Butter Oatmeal Sandwich Cookies with Marshmallow Crème Filling

Name: Peanut Butter Oatmeal Sandwich Cookies with Marshmallow Crème Filling
Ingredients: 1 stick (1/2 c.) unsalted butter, softened; 3/4 c. firmly packed light brown sugar; 1/4 c. granulated sugar; 1 tbsp. vanilla extract; 1/2 c. creamy peanut butter; 1 large egg; 3/4 c. quick oats; 1 c. all-purpose flour; 1/2 tsp. baking powder; 1/2 tsp. baking soda; 1/2 tsp. cinnamon; 1/8 tsp. nutmeg; 1/2 tsp. salt; 7 oz. jar Marshmallow Fluff; 1/2 c. (1 stick) unsalted butter, softened; 1/3 c. powdered sugar; 1/8 tsp. salt