How to Make It
Using tweezers or a small, sharp knife, carefully remove and discard pistil from each zucchini blossom. Trim stems to about 1 1/2 inches. Place mozzarella and prosciutto strips on a paper towel–lined baking sheet. Sprinkle mozzarella with 1/4 teaspoon salt. Let stand 10 minutes.
Pat mozzarella and prosciutto strips with paper towels to remove excess moisture. Carefully stuff zucchini blossoms with 1 strip each of mozzarella and prosciutto. Lightly twist flower at the top to help close.
Pour olive oil to a depth of 1/4 inch in a large cast-iron skillet; heat over medium-high to 350°F. Whisk together egg and 1/4 teaspoon salt in a medium bowl. Stir together flour and baking powder in a small bowl. Gradually whisk half of flour mixture into egg mixture. Add remaining flour mixture alternately with seltzer water, whisking until batter is smooth.
Dip 4 stuffed zucchini blossoms into batter, coating completely and allowing excess to drip off. Fry in hot oil, turning occasionally, until golden brown, 4 to 5 minutes. Transfer to a paper towel–lined baking sheet, and sprinkle with 1/8 teaspoon salt. Repeat with remaining zucchini blossoms, batter, and salt. Serve immediately.