Preheat your toaster oven to 350°F (175°C).
In a large mixing bowl, combine the softened unsalted butter, brown sugar, and granulated sugar. Cream until light and fluffy.
Add the large egg and vanilla extract, mixing until well incorporated and smooth.
In a separate bowl, whisk together the old fashioned rolled oats, all-purpose flour, baking soda, ground cinnamon, and kosher salt.
Gradually add the dry mixture into the wet mixture, stirring until just combined.
Line your baking sheet with parchment paper. Drop tablespoons of cookie dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for about 10-12 minutes until the edges are golden and the centers look set.
Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack.
Prepare the marshmallow topping by mixing the powdered sugar, marshmallow creme, vegetable shortening, and half and half until smooth.
Spread the marshmallow topping generously over each cookie.
Serve and enjoy your delicious Toaster Oven Oatmeal Cookies!
