In a bowl, cream together butter, brown sugar and white sugar. Beat in egg and vanilla extract.
In another bowl, combine oats, flour, baking soda, salt and cinnamon. Stir oat mixture into sugar mixture. Then mix in M&M's®.
Drop spoonfuls of cookie dough onto cookie sheets.
Bake for 10 minutes, or until done.
Remove from oven and slightly flatten cookies with a spatula. Allow cookies to cool on sheets for 5 minutes. Then transfer cookies onto a wire rack to cool completely.
Marshmallow Buttercream Filling
In a bowl, cream butter and then beat in confectioners' sugar and vanilla. Fold in marshmallow creme until incorporated.
Spoon a tablespoon of filling onto a cookie and top it with another cookie. Repeat with remaining cookies.
Enjoy!
Recipe by Sugar, Spice and Family Life at https://sugarspiceandfamilylife.com/2015/03/mm-oatmeal-cookie-sandwiches-recipe.html