Cut and squeeze the limes. You need half a cup of lime juice; this can take more or fewer than 6 limes.
In a blender, mix the condensed milk, evaporated milk, lime juice and vanilla.
Lay a layer of Maria biscuits in a baking dish or cake dish, making sure the bottom is fully covered. Break biscuits into pieces to fill the gaps.
Add a layer of the mixture from the blender.
Keep going until you have 2 or 3 layers of biscuits, finishing with the mixture on top.
Optionally, grate a Maria biscuit over the top.
Cover with plastic wrap. Chill for at least 30 minutes and serve as a dessert.
For the best result, leave it in the fridge overnight.