In a large bowl, whisk together flour and salt. Add cold butter, tossing to coat. Using a pastry blender or 2 forks, cut in butter until mixture is crumbly. Make a well in center.
In a small bowl, whisk together egg and 3 tablespoons (45 grams) ice water. Add egg mixture to flour mixture. Working one small section at a time, stir with a fork until mixture begins to become hydrated; start to gently knead dough together in bowl by folding it over on itself. Add up to remaining 2 tablespoons (30 grams) ice water, about 1 tablespoon (15 grams) at a time, until dough comes together. Shape into a disk, and wrap tightly in plastic wrap. Refrigerate for at least 1 hour.