In a large mixing bowl, combine the flour, butter, sugar, and salt. With your fingertips, rub the mixture together, breaking up the butter, until the mixture has formed peasized crumbs. Add the creme fraiche and the vanilla and mix together just until a dough forms. Gather the dough into a ball and flatten it into a disk. Wrap with plastic wrap and refrigerate for at least 2 hours. Preheat the oven to 400 degrees. In a food processor, pulse the guava paste with the lemon juice until a smooth paste forms. On a lightly floured surface, roll the pastry out to 1/4inch thickness and cut out thirty 2 x 2inch squares. Place 1/2 teaspoon of the guava paste in the center of each square. Dab a little beaten egg on the inside of one corner and on the outside of the opposing corner. Bring the two edges together until they meet and slightly overlap with the eggdabbed surfaces together, covering most of the paste. Press lightly to seal. Transfer the squares to a lightly oiled baking sheet and bake for about 25 minutes, or until the squares are just golden. Cool on racks and serve warm or at room temperature.