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 ¼ of 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.