Mini Chocolate Chip Cookie Sandwiches

 cup (1.5 sticks) unsalted butter, softened to room temperature

dark brown sugar

 cup granulated sugar

 egg, at room temperature

 teaspoon vanilla extract

 cups all-purpose flour

 teaspoons cornstarch

 teaspoon baking soda

 teaspoon salt

and 1/4 cup semi-sweet chocolate chips

frosting of choice (I used canned dark chocolate frosting this time, but next time it will be my Homemade Milk Chocolate Frosting)

assorted sprinkles