For the Tempura Batter: In a mixing bowl, blend the dry ingredients. Stir in the egg, and while stirring, pour in the seltzer, 1/2 cup at a time. Stir to completely combine ingredients. Add enough seltzer so that the blend is the consistency of unwhipped heavy cream. Stir in a few ice cubes to keep the batter very cold.
Preheat oil to 350 degrees F. Dip softshell crab into tempura batter. Fry crab until it turns golden brown. Remove and place on paper towels to drain. Cut into thirds. Place all ingredients on bamboo mat in the following order: nori, rice, crab, avocado, and spicy mayonnaise and roll in a jelly roll fashion.
Place all ingredients except oil in blender. On medium speed, slowly add the oil to emulsify into mayonnaise.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.