Preheat the oven to 350℉ and line two baking sheets with parchment paper or silicone mats and set aside for later.
In a large mixing bowl, combine the carrot cake mix, baking powder, and spices. Then using a hand mixer, add the melted butter and eggs to the bowl and beat on low until fully incorporated. You can alternatively mix everything with a rubber spatula if you do not have a mixer.
Fold in the grated carrots, rolled oats, and chopped walnuts.
Use a small cookie scoop or a spoon to drop 1 ½ tablespoon sized balls onto the baking sheets. Space them a few inches apart to leave room for the cookies to spread.
Bake at 350℉ for 10-12 minutes, or until the tops are set and it reaches desired doneness.
Allow the cookies to cool in the pan for 3 minutes before transferring to a wire cooling rack to cool completely.
As the cookies bake, you can prepare the frosting. In a medium mixing bowl using a hand mixer, beat the butter and cream cheese until smooth and creamy. Add the cornstarch, vanilla extract, and salt and mix until combined. Then add the powdered sugar a little at a time and mix again until well incorporated and you have a smooth, easily spreadable frosting. If you want a stiffer frosting without adding more sweetness, add more cornstarch 1 tablespoon at a time until it reaches desired consistency.
Once the cookies have cooled, you can use a piping bag or an icing spatula and put the icing on the bottom of a cookie. Then grab another cookie to place on top and you have a cookie sandwich! Repeat with the rest of the cookies.