Preheat the oven to 375 degrees Fahrenheit

Beat butter and sugars together in a large bowl with an electric mixer on medium speed until combined. Beat in eggs, one at a time, until incorporated; stir in vanilla

Stir flour, baking soda, and salt together in a bowl; Gradually add to the butter mixture, stirring until combined. Stir in oats and mix to thoroughly combine.

It’s not essential, but I like to chill the dough for 30–45 minutes to make it easier to scoop evenly sized balls.

Roll the dough into walnut-sized balls and place them on parchment-lined baking sheets. Bake in a preheated oven until lightly browned, about 8 to 9 minutes, though this can vary depending on the size of the dough balls and your oven—mine took 15. Remove when the edges turn light brown, let them cool on the baking sheet for 2 to 3 minutes as they’ll continue cooking, then transfer to a wire rack. Cool completely; they should be chewy, not rock hard, once cooled.

To make the cream filling: Beat butter and shortening on medium-high speed, until pale and fluffy, about 3 minutes. Add powdered sugar, beat on low speed until combined. Beat in marshmallow cream until smooth.

Spread desired amount of cream on half of cooled cookies,top with remaining half of cookies.