In a small bowl, mix the rice vinegar and sugar (microwave briefly to help dissolve). Add the mixture to the cooked rice and gently fold using a wooden spoon or spatula. Set aside to cool.
2 tablespoons rice vinegar, 1 teaspoon sugar, 2 cups Japanese sushi rice
To straighten the shrimp, make 5–6 shallow cuts along the inner curve. Flip them slit-side down on a cutting board and gently press along the back to flatten. Lightly dust each shrimp with flour.
10 large shrimp
In a bowl, whisk together flour, cornstarch, egg, and ice-cold water. Use chopsticks and don’t overmix—it's okay if the batter is slightly lumpy.
⅔ cup all-purpose flour, ⅓ cup cornstarch, 1 large egg, 1 cup very cold water
Heat oil in a deep pan over medium-high heat. Drop in a bit of batter—if it floats and sizzles, it's ready. Dip each shrimp in the batter, letting excess drip off, then fry in batches for 2–3 minutes until golden. Drain on paper towels and let cool slightly.
Oil
Place a bamboo sushi mat on a clean surface and lay one nori sheet on top, shiny side down. Wet your hands and gently spread a thin, even layer of rice over the nori, leaving a 1½-inch strip uncovered at the top.
Lay cucumber, avocado, and 2 shrimp tempura in a line near the bottom of the rice. Position the shrimp so tails stick out at both ends.
½ cucumber, ½ avocado
Roll tightly from the bottom, using the mat to press gently but firmly as you go. Wet the bare strip of nori to seal.
2 seaweed nori sheets
Using a sharp, wet knife, slice the roll into even pieces, cleaning the blade between cuts for best results.
Sprinkle with black sesame seeds and drizzle with sriracha mayo. Serve with soy sauce on the side.
Black sesame seeds, Sriracha mayonnaise