In a large bowl, use an electric mixer to beat the butter and sugar until fluffy and creamy. Add the egg yolks and mix. Add the brandy, vanilla and lemon zest; mix well and set aside.
In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt. Add the dry mixture to the butter mixture and work it together with your hands until all is combined and the dough is soft. Do not add any extra flour. You may wish to add a few drops of milk if the dough appears crumbly. Cover and chill for 2 hours.
Preheat the oven to 325°F. Line baking sheets with parchment paper.
Working with half of the dough at a time (keep the other half refrigerated) on a floured work surface, roll out the dough to a thickness of ¼-inch. Cut into 2-inch rounds and transfer to the prepared baking sheets. Repeat rolling and cutting with the 2nd half of the chilled dough. Bake for 12 to 15 minutes or until done. The cookies should be dry but not brown. Transfer to racks and cool.
When the cookies are completely cool, match them up into pairs of like-sizes. Turn one cookie over and spread a dollop of dulce de leche onto the flat part of the cookie. Place its partner on top (flat side down) and gently press so that the caramel comes to the edges. Roll the edges in coconut and place them on a rack to set. Continue with the rest of the cookie pairs.
When ready to serve, sift a light layer of powdered sugar over the tops of the cookies and place them on a platter.
Notes
Make sure that your room temperature butter is pretty soft. If it's too firm, the cookie dough will tend to be more crumbly.
Don't be afraid to use the brandy! It does NOT give a liquor taste to these cookies. It works well with the lemon and vanilla, and I think it adds an element of flavor that wouldn't be there if you left it out. If you have to leave it out, be sure to add 2 tablespoons of milk in its place.
Be sure to use 'grated' coconut and not shredded coconut. I am able to find it in a bulk bin at Whole Foods. If you cannot find it, go ahead and whir your shredded coconut in the food processor to create much smaller pieces.
Rounds are the typical shape used for these cookies. Try using different shaped round cutters. I made some 2-inch and then made some much smaller ones too.
Take the time to make homemade dulce de leche- it's so easy to do and you'll be happy you did! Instructions HERE.