Make the tempura batter just before dipping the crabs. The bubbles in the soda water help keep the tempura light and crispy.
Pour oil to a depth of 1 inch into a large Dutch oven, and heat to 350°.
In a medium bowl, combine flour and salt. Stir yolk into soda water in a 2-cup measure until blended; pour over flour mixture, and, using a large fork, gently combine (the batter should remain slightly lumpy and should have the consistency of heavy cream).
Dip 2 crabs into batter, letting excess batter drip back into bowl, and carefully slip into the hot oil, top-side down. Fry 1 1/2 minutes per side, until golden brown and crisp. Drain on paper towels, then transfer to a baking sheet, and keep warm in a 200° oven. Repeat with remaining oil and crabs.
Cut crabs in half. Serve with Asian Slaw and Crab Dipping Sauce. Garnish, if desired.
Wine note: "A crisp bubbly is traditionally a beautiful bedfellow for preparations involving frying, seafood, or soy sauce. This dish manages to combine all three, making a bubbly such as Zardetto Prosecco ($10) a no-brainer. The prosecco's clean, tart flavor whisks away the tempura's greasy baggage like a bellhop at the Four Seasons." --Mark Oldman