Heat the oil in a deep skillet or fryer to 350–375°F (175–190°C).

In a mixing bowl, combine mayonnaise, sriracha, lemon juice, brown sugar, and garlic powder. Whisk until smooth and creamy.

Pat shrimp dry with paper towels. Season with salt and pepper, and slice each shrimp in half lengthwise.

In a bowl, whisk together flour, cornstarch, baking powder, and salt. Slowly add cold beer or carbonated water to make a slightly lumpy batter.

Dip each shrimp into the tempura batter, allowing excess to drip off for an even coating.

Carefully lower battered shrimp into hot oil in small batches, frying for about 1-2 minutes until golden brown.

Use a slotted spoon to remove fried shrimp and drain on paper towels. Garnish with chopped scallions.

Serve immediately with sriracha aioli for dipping.