In a large mixing bowl using an electric mixer (or using a stand mixer) beat butter by itself until creamy.

Add powdered sugar, brown sugar, cornstarch, vanilla extract, and salt and beat until creamy and well-combined.

Gradually add flour (I usually add in about 4 parts) and stir until combined.

Divide dough in half and form each half into a disk. Cover with plastic wrap and transfer to the refrigerator to chill for at least 30 minutes and up to 5 days.

Once your dough has chilled, preheat oven to 375 F(190C) and line a baking sheet with parchment paper. Set aside.

Lightly dust a clean surface with flour and, working with one disk at a time, roll dough to be ⅛” thick.

Use a round cookie cutter (I like to use 1 ¾” but 2” would also work) to cut into circles. Place on prepared baking sheet, spacing at least 1” apart.

Transfer to 375F (190C) oven and bake for 8-9 minutes or until edges are just beginning to turn light golden brown.

Allow cookies to cool completely on baking sheet before removing and filling. Repeat with remaining dough until all cookies have been baked (never place cookie dough on a hot baking sheet!). Make sure to re-gather and re-roll out the dough scraps to get as many cookies as possible!

Once cookies have cooled, portion 1 ½ teaspoons of dulce de leche onto the bottom of one cookie and sandwich with another. If desired, you can roll the sides of the cookie through desiccated coconut (squeeze the “sandwich” so the dulce de leche reaches the sides and the coconut sticks to it) or dust the tops of the cookies with powdered sugar.