1. In a medium bowl, whisk together flour, two tablespoons granulated sugar, cornstarch, guar gum, and half a teaspoon of salt.
2. Transfer flour mixture to a food processor fitted with the S-blade. Add butter and pulse until crumbly.
3. In a small bowl, whisk together the apple cider vinegar, lemon juice, and one egg until foamy and mixture holds together. Add to food processor and pulse to combine.
4. Shape the dough into a flat disk, wrap in plastic wrap, and refrigerate eight hours or overnight.