Dry shrimp with paper towels. Spread on a large plate without overlapping them. Drizzle sake and grind salt and black pepper over top of shrimp evenly. Let it sit at room temperature to marinate for 10 to 15 minutes.

While marinating shrimp, prepare the vegetables. Scoop avocado from rind with a spoon and add it to a medium size bowl. Add purple onion, pepper, garlic, lime juice and 1/2 teaspoon salt. Smash and mix well with a potato masher.

Add cornstarch to the shrimp and mix well by hand. The shrimp should be evenly coated with a thin, starchy mixture.

Heat vegetable oil in a large nonstick skillet over medium heat. When oil is warm, spread shrimp one by one without overlapping them. Cook until the bottom side turns golden brown, 1 to 2 minutes. Flip with a pair of chopsticks (or tongs) to cook the other side, until the bottom side turns golden and the shrimp are fully curled, 1 to 2 minutes. Transfer to a plate. If your skillet is not big enough, cook the shrimp in two batches.

To assemble the sandwich, spread the guacamole on the bottom bun, place shrimp on top, garnish with chili flakes and sesame seeds, top with the top bun and pierce with a skewer.

Serve warm or cold.