Make the dipping sauce: Combine all ingredients in a bowl. Set aside to allow the flavors to develop.
Butterfly the shrimp by cutting down the back, being careful not to cut all the way through. Open the shrimp like a book and rinse well with cold water.
Make the Tempura batter: Put 1 cup of rice flour in a bowl and pour in the seltzer. Stir with a whisk to get out all the lumps. Add the egg yolk and blend it in well. The batter should be the consistency of heavy cream. Flavor with sesame oil, if using.
Heat about 2 inches of vegetable oil to 375 degrees F in a wok or deep fryer. Dry the shrimp well. Dust the shrimp in flour to soak up any remaining moisture, shake off excess. Dip the shrimp into the batter one by one. Drop 4 or 5 pieces at a time in the hot oil. Do not overcrowd the pan. Fry until golden brown, turning once, about 3 minutes. To keep the oil clean between batches, skim off the small bits of batter that float in the oil. Remove the fried shrimp from the oil and drain on paper towels; season them with salt. Serve with the dipping sauce.