Step 1: Preheat Your Toaster Oven. Set your toaster oven to 350°F (175°C).
Step 2: Cream the Butter and Sugars. Beat together the unsalted butter, brown sugar, and granulated sugar until light and fluffy.
Step 3: Add the Egg and Vanilla. Mix in the large egg and vanilla extract until well combined.
Step 4: Combine Dry Ingredients. In a separate bowl, whisk together the rolled oats, all-purpose flour, baking soda, ground cinnamon, and kosher salt.
Step 5: Mix It All Together. Gradually add the dry mixture to the wet mixture, stirring until just combined.
Step 6: Incorporate the Marshmallow Crème. Gently fold in the marshmallow crème and vegetable shortening.
Step 7: Scoop the Cookie Dough. Portion out the dough onto the prepared baking sheet, spacing them about 2 inches apart.
Step 8: Bake. Bake for about 10-12 minutes, or until the edges are golden brown and centers are just set.
Step 9: Cool and Enjoy. Let the cookies cool on the baking sheet before transferring to a wire rack.
