Sift flour, cornstarch, baking soda, and baking powder. Set aside.
Make sure your butter is very soft! Not melted, not warm, just perfectly soft. This will achieve the dough texture we need. Cream the soft butter and sugar for 5 minutes.
Add egg yolks, vanilla extract, and lime zest.
Add all the dry ingredients at once and mix until completely incorporated.*
Roll dough out (in portions so it's easier to work with) on a lightly floured surface**, to a 1/4 inch and cut into circles. Place on an unlined cookie sheet, no need to grease it. Also, you can place cookies pretty close together - they won't spread at all, so make sure you're cutting them out the size you're going to want the baked cookie to be.
Bake for 10-12 minutes and don't let them brown. Cool before filling.
Slather or pipe dulce de leche on the underside of a cookie and sandwich using another cookie. Press together so the dulce de leche slightly seeps out and roll the edges in shredded coconut.
Store in an airtight container.
Notes
*Note: You'll think that it's never going to incorporate, you're going to want to add some water - don't! Let it mix for a good 5 to 10 minutes and I promise, it will come together.
**Note: If you can roll out the dough with NO flour at all, even better. When working quickly with this dry dough, you will only need to flour your rolling pin, avoiding incorporating more flour into the already dry dough.
Servings: 30medium sandwich cookies, depending on size