Carefully rinse and dry the squash blossoms, if needed, to remove any dirt. Remove the stamens and discard.
12 fresh zucchini blossoms
In a medium bowl, stir together the goat cheese, gruyere cheese, and egg yolk. Transfer to a piping bag, pastry bag, or large Ziploc bag with the tip cut off of one corner.
1 (4-ounce) package garlic & herb goat cheese, ¼ cup freshly grated gruyere cheese, 1 large egg yolk
Pipe about 1 tablespoon of the goat cheese filling into each zucchini blossom, being careful not to overfill and burst the flowers. Refrigerate for 30 minutes.
When ready to begin frying, whisk flour, cornstarch, baking powder, and salt in a medium mixing bowl to combine. Whisk in ice water to form a smooth batter about the same thickness as a pancake batter. Try not to overmix the batter and be sure to use ice-cold water to prevent help prevent the gluten in the flour from developing.
1 cup all-purpose flour, ½ cup cornstarch, 1 ½ teaspoons baking powder, ¼ teaspoon salt, ¼ cup ice cold water
Heat about 1-inch of oil in a skillet or frying pan until it reaches 350°F on an instant digital thermometer. Dip each filled blossom into the batter, letting any excess batter drip off, then fry for 1-2 minutes, flipping halfway through, until a pale golden brown. Only fry 3-4 at a time so as not to crowd the pan.
2 cups vegetable oil
Carefully transfer the fried zucchini blossoms to a wire rack set over a baking sheet lined with paper towels to drain.