Preheat the oven to 400°. Line a large rimmed baking sheet with parchment paper. Cut the dough into 4 equal rectangles. Working with 1 rectangle at a time and keeping the rest refrigerated, roll out the dough on a lightly floured surface to a 10-by-12-inch rectangle, about 1/16 inch thick. Transfer to the prepared baking sheet. Brush with the beaten egg and sprinkle with sea salt. Let stand for 3 minutes to let the egg wash dry. With a pastry wheel, cut the rectangle into 2-inch squares. Separate the squares, leaving about 1/2 inch between them. Top the squares with another sheet of parchment paper and another baking sheet. Bake for 25 minutes, until the crackers are golden brown. Transfer to a rack to cool. Repeat with the remaining dough, egg wash and salt.