1. Preheat oven to 350°.
2. Weigh or lightly spoon flours and potato starch into dry measuring cups; level with a knife. Combine flours, potato starch, sesame seeds, and salt in a large bowl; beat with a mixer at medium speed until blended. Add milk, coconut oil, and egg; stir until a soft dough forms.
3. Shape dough into a ball. Cover a large baking sheet with parchment paper. Place dough in center of paper; gently press dough into a square. Cover dough with an additional sheet of parchment paper. Roll dough, still covered, into a 18 x 12-inch rectangle. Remove top sheet of paper; cut dough into 36 (2-inch) squares. Separate squares on pan.
4. Bake at 350° for 18 minutes or until crackers are golden brown and crispy. Cool 20 minutes on pan.