Special equipment: a deep-frying thermometer
For the tartar sauce slaw: Whisk together the cornichons, mayonnaise, capers, mustard, sugar, lemon juice, onion and some salt and pepper in a large bowl. Add the cabbage and toss to coat. Cover and refrigerate for 30 minutes.
For the Old Bay potato chips: Meanwhile, preheat the oven to 300 degrees F. Spread the potato chips in an even layer on a baking sheet and bake, just to warm, 2 minutes. Put the Old Bay in a small mesh strainer and tap evenly over the chips; toss the chips lightly.
For the soft-shell crabs: Set a rack in a baking sheet. Heat 3 to 4 inches of oil in a Dutch oven over medium heat until it registers 375 degrees F on a deep-frying thermometer. Whisk together 2 cups of the rice flour and 2 cups water until smooth, then season with salt and pepper and some hot sauce if using.
Put the remaining 1 cup rice flour in a shallow dish and season with some salt and pepper. Sprinkle the crabs with salt and pepper.
Dredge 2 crabs in flour and tap off the excess. Dip in the tempura batter, letting any excess run off. Fry until golden brown, crispy and just cooked through, turning occasionally, about 5 minutes. Drain on the rack. Repeat with the remaining 2 crabs.
For the sandwiches: Put some of the slaw on the bottom of each bun and top with a fried crab. Add a bit more slaw and a tomato slice and top with chips. Close sandwiches with the tops of the buns and serve immediately.