Keep the squash blossoms and sparkling water chilled in the fridge until the filling and batter are ready. Zest and juice one lemon, keeping the juice and zest in separate bowls for later use.
To make the filling, add the goat cheese, heavy cream, chopped herbs, kosher salt, pepper, and the zest of half a lemon to a medium mixing bowl and whisk together until smooth and thick, like a frosting.
Fit a large gallon ziplock bag or icing bag into a large glass or mug, allowing the overhang of the bag to fold over the sides of the glass, making it easier to transfer the filling into the bag. Transfer the filling deep into the bag, then remove the bag from the glass, and snip a ¼-½ inch hole in the tip.
Take the chilled squash blossoms and fill each one: Peel the petals back and place the tip of the bag as close to the stamen or interior base of the blossom as possible. Fill with roughly 2 teaspoons of the goat cheese filling and then use the long petals to twist and seal the filling inside. Fill all the blossoms and place them back in the fridge while you make the batter.
In a large or wide mixing bowl, whisk the flour, corn starch, and baking powder together until completely combined. Slowly pour in 1 cup of chilled sparkling water and 2 Tbsp of lemon juice, gently whisking to combine. Be sure not to over whisk the batter—the flour will slowly hydrate itself and de-clump as you gently stir. The final batter should be a thin pancake batter consistency.
Fill a cast iron pan or small wok with 2-3 inches height of frying oil and place it on medium high heat, until the oil reaches about 365 degrees F. You’ll know the oil is ready when you drip some batter in and it immediately bubbles up and rises to the surface of the oil. Keep oil on hand in case the frying oil gets too hot (if the batter immediately turns brown when submersed).
Dip the filled squash blossoms in the batter and let it coat the entire blossom, allowing excess batter to drip back into the bowl of batter. Place battered blossoms into the oil, making sure to only add 2-3 at a time, depending on the size of the frying vessel. Fry the blossoms for 2-3 minutes on each side and then transfer to a paper towel lined plate.
Garnish the fried squash blossoms with more lemon zest and fresh chopped herbs.