place butter in a plate, and mash with a fork until soft and creamy.
sift sugar over butter, and keep on mashing.
gradually add lemon juice until it forms a soft paste. Once it is soft, add more lemon juice to taste. Be carefull not to add too much, since the paste could turn liquid and not work.
once happy with the taste, place in a bowl.
take two vanilla cookies, and spread lemon paste on one, then place the other one on top of it, forming a sandwich.
repeat step 5 with all cookies. The paste should be about as thick as in the picture.
place cookie sandwiches on a sheet pan, and chill in the freezer for about 15 minutes.
with a spoon, coat the top of the sandwich with chocolate. Personally, I like having only a light layer of chocolate over the top cookie, but if you want, you can coat the whole sanwich with chocolate.
once all sandwiches are coated with chocolate, freeze them until the chocolate hardens, then store in the fridge until serving. (tip from my grandma: prepare them a day ahead, and chill overnight. they will be softer and the flavors will be enhanced.)
serve, and enjoy!