In a large mixing bowl, sift together the flour, powdered sugar, almond flour, and salt. Briefly mix to combine.
Add the cold butter cut into cubes to the flour mixture. Using your fingers, coat each piece of butter with flour by gently crushing them. Then, work the butter between your fingers until you achieve a sandy and homogeneous texture (about 3 to 4 minutes).
Make a well in the center of the flour mixture.
Pour the beaten egg into the well. Using a fork, gradually incorporate the flour into the egg until you obtain a crumbly and flaky dough.
Transfer the dough onto a clean, unfloured work surface.
Using the palm of your hand, flatten the dough by pressing it out in front of you, then gather it with a pastry scraper or a spatula. Repeat the operation 6 to 8 times until you obtain a soft and slightly sticky dough.
Shape the dough into a log 25 cm long and 3 to 4 cm in diameter. For a crunchy touch, roll the log in demerara sugar before wrapping it.
Wrap the log tightly in plastic wrap, twisting the ends to seal. Roll the log under the palms of your hands to smooth and even its shape.
Refrigerate for at least 3 hours (ideally overnight) for a firm texture.
Line two baking sheets with parchment paper.
Using a sharp chef's knife, cut the ends of the log for a clean finish, then slice into rounds 6 mm thick.
Place the sablés on the sheets and freeze for 30 minutes before baking
Bake the sheets until the edges are lightly golden
