How to Make It
Preheat the oven to 400°. In a food processor, combine the flours, sugar and 1/2 teaspoon of salt and pulse for 15 seconds. Add the butter and pulse for another 15 seconds, until a coarse meal forms. Add 1/2 cup of milk and pulse until the dough just comes together, about 30 seconds; add the extra 1/4 cup of milk if necessary. Gather the dough into a ball and wrap in plastic. Refrigerate for 15 minutes.
Line 2 baking sheets with parchment paper and divide the chilled dough into 4 even pieces. Dust a work surface and rolling pin with flour. Roll out 2 pieces of the dough as thinly as possible into 13-by-8-inch oblong rectangles. Transfer to the prepared baking sheets. Prick the dough all over with a fork. Brush lightly with some of the egg white and sprinkle with salt. Bake the crackers in the center of the oven for about 14 minutes, until golden brown and crisp; shift the sheets halfway through baking. Transfer the crackers to a rack and let cool for 15 minutes. Repeat with the remaining pieces of dough. Break the crackers into shards or serve whole.