These thin, tasty crackers are super-crunchy and versatile. Slideshow: Great Cracker Recipes
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.
The crackers can be stored in an airtight container for up to 2 days.