How to Make It
In a large saucepan of boiling water, cook the soba noodles until al dente, about 4 minutes. Drain and rinse under cold water until cooled. Shake out the excess water and transfer the noodles to a large bowl; drizzle with oil and toss to coat.
Rinse out the saucepan. Add the water and kombu and bring to a simmer over moderate heat. Discard the kombu. Add the bonito and remove the saucepan from the heat. Let stand until the broth is well-flavored, about 3 minutes. Strain the dashi through a fine sieve into a clean saucepan. Stir in the shoyu, light shoyu and sugar and season the broth with salt; keep hot over low heat.
In a large saucepan, heat 1 inch of oil to 365°. Set a rack over a baking sheet. In a medium bowl, whisk the rice flour with the baking powder, baking soda and a pinch of salt. Gently whisk in 1 cup of the club soda just until a smooth batter forms; add more club soda if the batter is too thick. Working close to the stove, dip 6 of the shrimp in the batter and add them to the hot oil. Fry until golden brown and just cooked through, 2 to 3 minutes. Using a slotted spoon, transfer the shrimp to the rack to drain. Repeat with the remaining shrimp.
Transfer the soba to bowls and top with the hot broth. Garnish with the shrimp tempura, radishes and scallions and serve.
Zimmern also likes to serve the soba chilled with julienned raw vegetables and a chilled dipping sauce made by mixing 1 cup of the strained (but not seasoned) dashi with 2 tablespoons of shoyu, 3/4 teaspoon of tamari and 1/2 teaspoon of sugar.