Step 1: Prepare the shrimp by making 3-4 shallow cuts on the underside of each to prevent curling during frying.
Step 2: In a bowl, whisk together ice-cold water and egg yolk, then gently stir in flour and baking powder until just combined (lumps are okay).
Step 3: Heat 2 inches of vegetable oil in a deep pot to 350°F (175°C).
Step 4: Dip shrimp in tempura batter, letting excess drip off, then fry in batches for 2-3 minutes until golden and crispy. Drain on paper towels.
Step 5: In a large pot, combine dashi broth, soy sauce, mirin, sugar, and salt. Bring to a simmer over medium heat.
Step 6: Cook udon noodles according to package directions, then drain and divide among 4 bowls.
Step 7: Ladle hot broth over noodles in each bowl.
Step 8: Top each bowl with 3 shrimp tempura, green onions, nori strips, and optional boiled egg halves.
Step 9: Serve immediately while tempura is still crispy.