In a blender or food processor, add all guava shells and half the syrup from one can. Process until it reaches a smooth consistency. Put puree in a medium saucepan and cook at low heat until reduced to a very thick consistency. Let guava cool and then fill into pastry bag. In a medium mixing bowl, add 4 ounces of water, 2 ounces of butter, and salt and mix until smooth. Then add flour and work well until all ingredients are incorporated. Continue to add water, a little at a time, working the dough until a medium-hard consistency is reached. Form dough into a ball and let rest for 15 minutes. Roll the dough into a flat square, about 1/2-inch thick.
Place the remaining butter in the middle of square and pat down by hand to form a square about one-half the size of the dough square. Fold over the 4 corners of the dough, 1 at a time, overlapping each other so that a sealed envelope is formed enclosing the butter. Dust with flour and roll out to form a rectangle about 1/4-inch thick. Now fold 1/3 of the rectangle dough towards the middle and the remaining 1/3 over those 2. Let dough rest for 15 minutes. Now repeat process above of rolling out and folding in 3, 2 more times, resting 15 minutes between each.
Preheat the oven to 350 degrees F.
Roll out a final time into a rectangle and fold 1/4of the dough on each side to the middle and then fold 1 more time down the middle to form 4 flaps in the fold. Let rest for 15 minutes. Roll out to a rectangle to about 1/8-inch thick. With a knife, cut the flattened dough into a 4-inch wide strip by length of your baking sheet. Lay strip on sheet pan lined with waxed paper and put filling lengthwise in the middle in continuous stream about 3/4-inch in diameter. Spray edges of strips with water and cover with another 4-inch strip of dough the same size as bottom. Press the edges together by hand very softly all the way around. Brush top with egg wash and bake for 35 minutes. Remove from oven and brush with a simple syrup over the top and bake for another 5 to 7 minutes. Remove from oven and cut crosswise into 16 pieces.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.