Chop breast + thighs into small bits. In a food processor, pulse together with salt, white pepper, onion & garlic powders, and sage until no more chunks and it sticks together. (Don’t overdo it—texture matters.)
Scoop 3-4 oz portions of that mixture and shape into thin patties. Line them on a parchment tray. Freeze for about 30 minutes so they firm up and are easier to handle.
Meanwhile, heat oil to about 325°F (1-1.5 inches deep). Medium heat; you want golden, not burnt.
Whisk together all the dry batter ingredients (flour, cornstarch, baking powder, salt, black pepper, paprika, chili powder, garlic powder). Coat patties in this dry mix; shake off excess.
To the leftover dry batter, add cold iced water + baking soda. Whisk to a thin wet batter. Dip each dry-coated patty into this wet batter so it’s completely coated.
Fry in batches—don’t overcrowd. Cook until patties are golden brown. Remove and let drain on a wire rack so oil doesn’t pool under them (crispier = good).
Toast the brioche buns with butter until golden. On the bottom bun, place a leaf of butter lettuce, then your crispy chicken patty. Top with whatever dressings you like (mayo? McChicken sauce? Totally optional). Crown with top bun.