Combine the flour, salt, baking powder, and sugar in a food processor fitted with the metal blade and process for 5 seconds.
Add the butter and pulse 10 times, or until the mixture is the consistency of fine meal.
With the machine running, add the water in a steady stream, taking about 10 seconds to do so.
Process for 1 minute. The dough will gather into a ball and form a mass that whirls around the blade. Feel the dough. It should be smooth, soft, elastic, and no longer sticky.
If necessary, adjust the texture with droplets of water or small amounts of flour, processing a few seconds after each addition. If you want to add seeds to the dough, knead them in by hand on your work surface.
Divide the dough into 24 pieces and shape into balls (3/4 ounce each).
Cover the balls of dough loosely with a kitchen towel and let stand for 30 minutes.
Next, the filling:
Place the cheese, salt, cayenne, egg, and egg white in the work bowl of a food processor fitted with the metal blade and pulse very rapidly 20 to 30 times, until the mixture just begins to gather into a mass.
Turn out onto a sheet of waxed paper or plastic wrap and pat into a 6 x 4–inch rectangle.
Cut the cheese mixture into 1-inch squares.
Adjust two oven racks to divide the oven into thirds and preheat the oven to 350°F
Line two 14 x 17–inch baking sheets with silicone baking pan liners or cooking parchment.
To shape the sambouseks, pat a ball of dough into a 4-inch circle on your unfloured work surface.
Place a square of the cheese mixture slightly below the center of the circle and pat it into a semicircular shape, leaving a bottom border of dough a scant 1/2 inch wide.
Fold the top half of the dough over the cheese and press the edges firmly to seal.
Use a fingertip to crimp the edge of the dough back on itself, forming a fluted border.
Place the sambousek on the prepared sheet.
Repeat with the remaining dough and cheese, placing 12 sambouseks about 2 inches apart on each sheet.
Paint the sambouseks with the egg wash.
Bake 30 minutes, or until golden brown.
Rotate the pans top to bottom and front to back once during baking to ensure even browning. Use a wide metal spatula to transfer the sambouseks to wire cooling racks, and serve warm or at room temperature.