Whisk together the egg and buttermilk in medium bowl. Add shrimp and refrigerate for 30 minutes.
Line a baking pan with parchment paper.
In large ziplock bag, combine cornstarch and panko bread crumbs.
Remove shrimp from egg/mixture 2-3 at a time, shake off excess, then drop into ziplock of crumbs and toss to coat. Remove to baking sheet. Continue until all shrimp have been coated. Add addtional corn starch and panko if required. Refrigerate shrimp for 30 minutes before frying if possible.
Heat oil in a deep skillet or wok to 350 degrees. Test with small piece of bread...should sizzle immediately when dropped in oil and turn golden brown within 30 seconds.
Fry shrimp 2-3 at a time for 1-2 minutes. They will only be a light golden brown, however do not overcook. Remove to drain on a brown paper bag or paper towels.
Once shrimp are cooked, gently toss in sauce, then serve over shredded lettuce/cabbage and sprinkle with scallions.