Combine 1 bag tempura batter mix with 3 quarts plus 2 cups ice water. (Mix second bag when needed.) Stir gently until only a few lumps remain; do not over mix. Pat shrimp dry and dust with dry tempura batter mix. Dip shrimp into tempura batter, coating completely; roll in panko. Drop shrimp gently into hot oil (375°F). Fry until golden-brown. Drain thoroughly. Serve with ponzu sauce for dipping.