1Soften the sticks of butter and join them to form a single block. Cover with plastic wrap and refrigerate.
2Place all the ingredients except for the butter in the food processor to mix.
3Remove from the food processor, knead by hand into a ball and make shallow intersecting incisions on the surface. Cover with a moist rag and refrigerate for 5 minutes.
4After this time, stretch the dough to form a 20-centimeter by 20-centimeter square. Stretch the edges outward to create “flaps.” Place the butter block in the middle. Close the flaps over the butter, overlapping one another as if closing a box.
5Using the rolling pin, stretch to a 20-centimeter by 40-centimeter rectangle.
6Fold the dough three times towards the center, then turn over and stretch lengthwise again. Fold three times again.
7Refrigerate for 30 minutes. Stretch and fold again. Do this process twice.
8Refrigerate overnight and repeat this process twice the next day, 8 times in total. Let it rest for 1 hour.
9Preheat the oven to 200 degrees Celsius.
10Cut 15- by 15-centimeter squares. Inside each one, cut out another square. Lift 2 of the 4 edges and twist around to create a border. Poke the center of each vol-au-vent with a fork to keep it from rising. Add some of the egg to secure the border in place, then cover the top part of the pastry, being careful not to cover the sides or the layers won’t rise.
11Place in the oven. For the first 10 minutes add a heavy flat object (a silpat, for example) on top of the vol-au-vents so they will rise evenly. Remove the silpat and continue baking for 10 to 15 minutes.
12Let the vol-au-vents cool and add your filling of choice, sweet or savory.