Roughly chop the shrimp meat into chunks and add to a food processor (preferably a mini-processor or your processor’s mini work bowl). Puree.
Add the egg white and run the processor until well incorporated. Move the mixture to a covered bowl in your refrigerator and chill thoroughly.
Once the shrimp puree has been thoroughly chilled, stir in the heavy cream and seasonings. The resulting consistency should be thick enough that it can be dolloped. If it’s too thin, use a whisk and whip it until it firms up.
Test the seasoning by putting a dollop of mousse in the middle of a small piece of heavy-duty plastic wrap. Fold the wrap over the dollop, taking care not to crush it. Twist the ends of the wrap closed and tie them off so you have a little bundle. Poach the bundle in boiling water until it puffs up, turns a light pink, and develops a firm but spongy texture (make sure it reaches 160°F). Unwrap the bundle and taste the mousse. Adjust the seasoning as desired.
Transfer the seasoned mousse to a piping bag or small zip-top bag (clip a corner off later to use it as a piping bag stand-in). Store in your refrigerator.
Heat the canola oil in a fryer or heavy Dutch oven to 375°F.
Fill your squash blossoms and return them to the refrigerator. See: How to Stuff Squash Blossoms Tutorial
. Keep in mind that the mousse will expand as it cooks, so underfill them slightly.
Just before you’re planning to serve the blossoms, combine the egg yolk, rice flour, 2 heavy pinches of salt, and sparkling water in a bowl. Depending on your rice flour, you may need to add more water to thin or more flour to thicken to get the right consistency. You want a thin batter that lightly coats food in an opaque layer.
Dip each blossom in the tempura batter and immediately transfer to the hot oil. Don’t fry more than five or six at a time, you don’t want to crowd them.
Turn each blossom as necessary to cook them on all sides. This batter crisps but doesn’t brown much, so use its texture as a cue to doneness rather than its color. When the blossoms start to puff, they’re probably done. (They’ll gently burst if you’ve overfilled them.) To be sure they’re done, cut one open and compare the texture/temperature of its filling with your test-dollop to be sure. If it’s undercooked, return the blossom halves to the oil and continue to cook them.
Remove the cooked blossoms from the oil and drain them on paper towels. Let the oil reheat to 375, then cook another batch, or cook other vegetables you’ve dipped in the batter.
Serve hot with dipping sauce on the side.