Hoisin Butter Sauce: In a medium, heavy saucepan, gently heat the hoisin and vinegar over medium-low heat. Whisk in the butter, 1 tablespoon at a time, adding each piece before the previous piece has been completely incorporated. Continue until all the butter is incorporated and the sauce coats the back of a spoon, removing the pan from the heat periodically to prevent the sauce from getting too hot and breaking. Keep warm while you prepare the squash blossoms.
Gently wash the squash blossoms in a sink of lightly salted water to remove any insects or dirt. Remove and dry.
With a thin knife, cut the squash almost in 1/2 lengthwise, starting at the bottom end and working toward the blossom, to open for better cooking.
In a medium bowl, combine the cream cheese, pepper jack, green onions, bell pepper, cilantro, garlic, lemon juice, salt, and white pepper, and mix well. Fold in the crabmeat. Transfer the mixture to a pastry bag without a tip. Pipe the crabmeat mixture into each squash blossom, about 1 1/2 tablespoons per blossom.
In a large, heavy pot, heat the oil to 350 degrees F.
To make the tempura batter, in a medium bowl, combine the cornstarch and Essence. Slowly add the cold soda water, whisking to make a thin batter. Add the ice to make the batter cold. Place the stuffed squash in the batter, 1 at a time, shaking to remove any excess. In batches, carefully drop into the hot oil and fry until golden, about 3 minutes. Remove and drain on paper towels. Season lightly with Essence, to taste.
Transfer squash blossoms to a large platter and serve hot with the Hoisin Butter Sauce.