Rinse the soft shell crabs under cold water to remove any residue. Pat them dry thoroughly with paper towels. Remove the face plate (optional) and the gills if your crabmonger hasn't already done so. Make 2 shallow diagonal slashes across the top of each crab to help them remain flat while frying.

Make batter

In a medium bowl whisk together the all-purpose flour, cornstarch, baking powder, salt, black pepper, and garlic powder until evenly combined. In a separate small bowl lightly beat the egg with the cold club soda (or ice water). Pour the wet mixture into the dry ingredients and whisk until just combined; a few small lumps are fine. Batter should be thick enough to coat the back of a spoon but still fluid. If using panko for extra crunch, place it in a shallow dish.

Heat oil

Pour oil into a deep skillet or small saucepan to a depth of about 1 to 1¼ inches (enough to shallow-deep fry). Heat over medium-high until the oil reaches 350°F (175°C). If you don't have a thermometer, test by dropping a small amount of batter into the oil; it should sizzle and rise to the surface fairly quickly.

Batter and optional panko

Working with one crab at a time, lightly dredge each crab in a thin dusting of flour (helps batter adhere), then dip into the batter, letting excess drip off. For extra texture, press the battered crab into the panko on both sides, gently pressing so crumbs adhere.

Fry crabs

Carefully lower the battered crab into the hot oil, presentation side down first. Fry 2 to 3 minutes per side until golden brown and crisp, maintaining oil temperature near 325–350°F (160–175°C). Avoid overcrowding the pan; fry in batches if necessary. Use tongs to turn the crab once; total cook time should be about 4–6 minutes per crab depending on size. Crabs are already cooked internally; you are crisping the shell and heating through.

Drain and season

Transfer fried crabs to a wire rack set over a baking sheet or to paper towels to drain excess oil. Immediately season lightly with a pinch of fine sea salt. Let rest 1 minute to firm up and cool to safe handling temperature.

Serve

Place crabs on a serving plate, garnish with lemon wedges and chopped parsley if using. Serve hot with the Sweet Mayo Sauce on the side for dipping.

Sweet Mayo Sauce

In a small bowl whisk together the mayonnaise, honey, sriracha or sweet chili sauce, rice vinegar (or lemon juice), soy sauce, and sesame oil until smooth. Taste and adjust seasoning with salt and black pepper. If you prefer a thinner sauce, whisk in 1–2 teaspoons of warm water. Refrigerate the sauce for at least 10 minutes to let flavors marry; bring to room temperature before serving if chilled.