In a large bowl, combine the butter, the flour and the sugar and mix everything together, using your hands, until a dough forms. Continue working the dough until it is smooth and doesn't stick to your hands anymore.
Once the dough is ready, form a ball and then flatten it slightly into a thick disc. Cut the disc into 4 equal parts.
Roll each part into a ball again and then roll the ball into a skinny log. Using a knife, cut the log into 18 equal parts and make little balls. (You could also roll the entire dough to 1/4" thick and use a 2" cookie cutter to cut the cookies. However, they will be thinner that way.)
Add the little balls to baking sheets lined with parchment paper and, using your thumb, flatten the balls slightly.
Bake your cookies for 15 minutes or until the bottom starts developing a golden color. (They will still be soft on the top! Don't worry!)
Let the cookies cool and set for at least 30 minutes.
Add the desired amount of dulce de leche to half of the cookies and sandwich them with the remaining cookies.
Finally, roll the cookies in confectioner's sugar.
