1, Prepare the crabs: Working with one crab at a time, flip it belly-side up and, with your fingers, pull off the apron (a visible flap opposite the eyes). Flip the crab back over and lift up the shell. On either side, you’ll see a feathery bit (the lungs); using your fingers, rip them out. Finally, with a sharp pair of scissors, make a straight cut just behind the eyes.
2, In a small bowl, stir together the flour, cornstarch, baking powder, turmeric, salt, and cayenne pepper. In a larger bowl, whisk together the egg, vinegar, and 1 bottle of the beer.
3, Using a rubber spatula, fold the flour mixture into the beer mixture. It should have the consistency of thin pancake batter and may be a little lumpy; do not overmix. Add more beer if needed to achieve the correct consistency. Cover and refrigerate until ready to use.
4, Pour the oil to a depth of 2 inches into a wok or high-sided pot and heat over high heat to 375°F on a deep-frying thermometer. Line a rimmed baking sheet with paper towels. Preheat the oven to 250°F. When the oil is ready, dip a crab into the batter, allowing the excess to drip off, and then, holding the crab by the shell flaps, carefully lower just the crab legs and body into the hot oil. Wait for 2 seconds, then drop the entire crab into the oil. Repeat with 2 more crabs. Cook for 6 minutes, until golden brown and crispy.
5, Using a spider, transfer the crab to the paper towel–lined baking sheet to drain, then transfer the baking sheet to the oven to keep warm. Let the oil return to temperature, skimming any bits of batter from the oil with a spider, then fry the remaining three crabs as described above, draining then briefly on a paper towel–lined baking sheet.
6, Cut each crab in half and arrange the pieces on a large platter, surrounded by the lettuce and rau ram. Place the sauce in a bowl alongside. To eat, wrap a piece of crab in a lettuce leaf with some rau ram and dip into the sauce.