How to Make It
Prepare the Oatmeal Cookies: Preheat oven to 375°F. Beat together butter and sugars with an electric mixer on medium speed until combined. Beat in eggs, 1 at a time, until incorporated; stir in vanilla. Stir together flour, baking soda, and salt in a bowl; gradually add to the butter mixture, stirring until combined. Stir in oats until incorporated.
Roll the dough into walnut sized balls, and place on baking sheets lined with parchment paper. Bake in preheated oven until browned, 8 to 9 minutes. Let cool on baking sheets 5 minutes before transferring to a wire rack. Cool completely.
Prepare the Crème Filling: Beat together butter and shortening on medium-high speed, until pale and fluffy, about 3 minutes. Add powdered sugar, and beat on low speed until combined. Beat in marshmallow crème until smooth.
Pipe or spread desired amount of crème onto half of cookies, and top with remaining half of cookies.