1. Combine flour, 1 tsp. salt, 1 tsp. pepper, and the cayenne in a large bowl. Pat crab pieces dry with paper towels and toss (in batches) with flour mixture. Remove crab and shake off excess flour. Set aside.
2. In a wok or large pot, heat 3 in. oil to 375°. Lay out paper towels for draining crab and garlic. Fry crab in batches (do not crowd wok) until golden, about 5 minutes per batch. Drain on paper towels.
3. Using the same hot oil, fry garlic until golden brown, 2 to 3 minutes. Remove with a slotted spoon and drain on paper towels. Set garlic aside; cool and discard oil.
4. Bring a large pot of water to a boil. Add 1 tbsp. of the salt and the spaghettini. Cook until tender to the bite, 5 to 10 minutes. Drain, transfer to a serving bowl, and toss with butter and half of the fried garlic. Cover and put in a warm place.
5. Heat a wok or pot large enough to hold all the crab over high heat. Add remaining 3 tbsp. oil, dried chiles, and ginger. Cook, stirring constantly, until fragrant, about 30 seconds. Add green onions, serrano chiles, and remaining 1/2 tsp. salt. Cook, stirring, until onions wilt, about 1 minute. Add sake and cook, stirring, until sake is reduced by about half. Stir in crab and cover. Cook until crab is heated through, about 3 minutes.
6. Remove lid and cook, stirring, until any liquid evaporates. Stir in basil, mint, cilantro, and remaining 1/2 tsp. pepper. Cook, stirring, until herbs have wilted. Stir in remaining fried garlic. Transfer crab to a warm platter and serve hot, with garlic noodles.
Note: Nutritional analysis is per serving.
Frying the crab first is optional, but it adds superb flavor and makes the crabmeat more tender. The results will be even more transcendent if you start with raw crab (get it from a fishmonger, and fry it a minute or two longer than cooked crab).
For a simpler version of this dish, skip steps 1 and 2 (frying the crab). Put 1 cup of the oil in a small saucepan over high heat and start recipe with step 3.