Preheat the toaster oven to 350°F. Spray a toaster oven sheet pan with nonstick cooking spray or line it with a small piece of parchment paper.

In a large mixing bowl, cream together ½ cup unsalted butter, ½ cup brown sugar, and 3 tablespoons granulated sugar with a hand mixer or stand mixer on medium speed until light and fluffy, about 3 minutes.

Add 1 large egg and ½ teaspoon vanilla extract to the butter-sugar mixture and mix just until combined, scraping down the bowl as needed.

In a medium bowl, whisk together 1½ cups old-fashioned rolled oats, ½ cup all-purpose flour, ½ teaspoon baking soda, ½ teaspoon ground cinnamon, and ½ teaspoon kosher salt.

Add the dry ingredients to the wet ingredients and mix until just combined. The dough will be thick and should hold together.

Use a tablespoon to portion dough into balls and place them on the prepared sheet pan (depending on pan size, about six cookies per batch). Leave space between cookies to spread.

Gently press each dough ball down to flatten slightly before baking.

Bake at 350°F for up to 15 minutes, checking at the 10-minute mark; bake until the tops are lightly browned. Toaster oven times and heat can vary, so watch closely.

Transfer baked cookies to a wire rack and let them cool completely. Repeat baking in batches until all dough is used.

Make the filling: in a bowl, beat 1½ cups powdered sugar, 3½ ounces marshmallow creme, and ⅓ cup vegetable shortening with an electric mixer until smooth.

Add 1 tablespoon half and half and blend until the filling is smooth and spreadable. Add additional half and half, a small amount at a time, if needed to reach the desired consistency.

Assemble cookies: spread or pipe the filling onto the flat side of one cooled cookie, top with a second cookie, and press gently to create a sandwich.