Beat butter with sugar (best to do this by hand). Add yolks, rum extract, vanilla, and greated lemon until well combined. With your hand, gently add in the flour and cornflour. 

With a wet hand, separate shotglass-width sized balls of dough. Flatten each with your hand, then place on the cookie sheet. Bake for maximum 7 minutes at 350°F.

Remove and let cool. 

Create sandwiches by filling 2 cookies with dulce de leche from a pastry bag.

Optional: Roll each cookie in coconut flakes.