In a big mixing bowl, add the softened butter, powdered sugar, vanilla extract, kosher salt, and egg yolks, and mix with an electric hand mixer for 2-3 minutes or until smooth and creamy.
Add the cornstarch, flour, and baking powder, and mix until the dough comes together.
Divide the dough in half cover each dough square with plastic wrap and let the dough rest in the fridge for at least an hour.
After allowing the dough to chill in the fridge, take one dough half out of the fridge. Place the dough between two parchment papers and roll the dough using a rolling pin to about ¼ inch thick.
Cut into circles with a round cutter, we use a small 4-centimeter one, but you can make them bigger.
Carefully transfer the cookies onto a parchment paper-lined baking sheet and bake in a 180c (350f) preheated oven for 10 minutes.
The cookies will look pale, but they will be ready after 10-12 minutes depending on their size.
Allow the cookies to cool to room temperature and fill each cookie with about 1 teaspoon of dulce de leche. Use a piping bag to do so or a teaspoon.
Dip the edges of each cookie in the shredded coconut and serve!