Instructions

For the dough: Whisk flour, yeast, and salt together in large bowl. Whisk room temperature eggs, melted butter, water, and sugar together in second bowl until sugar has dissolved.
Using rubber spatula, gently fold egg mixture into flour mixture, scraping up dry flour from bottom of bowl, until cohesive dough starts to form and no dry flour remains. Cover bowl tightly with plastic wrap and let dough rest for 10 minutes.
Using greased bowl scraper (or your fingertips), fold dough over itself by gently lifting and folding edge of dough toward middle. Turn bowl 90 degrees and fold dough again; repeat turning bowl and folding dough 2 more times (total of 4 folds). Cover tightly with plastic and let rise for 30 minutes. Repeat folding and rising every 30 minutes, 3 more times. After fourth set of folds, cover bowl tightly with plastic and refrigerate for at least 16 hours or up to 48 hours.
Line 2 rimmed baking sheets with parchment paper. Transfer dough to well-floured counter and divide into 8 equal portions (about 2½ ounces or 71 grams each). Working with 1 piece of dough at a time (keep remaining pieces covered), form into rough ball by stretching dough around your thumbs and pinching edges together so that top is smooth. Place ball seam side down on clean counter and, using your cupped hand, drag in small circles until dough feels taut and round. Repeat with remaining pieces of dough.
Evenly space 4 rounds seam side down on each prepared sheet, about 3 inches apart. Cover loosely with greased plastic and let rise until rounds double in size and dough springs back minimally when poked gently with your knuckle, 1½ to 2 hours.
For the filling: Toss zucchini with ½ teaspoon salt and spread over paper towel–lined plate. Let zucchini sit for 30 minutes, then gently blot dry with paper towels; set aside. Combine ricotta, mozzarella, 2 tablespoons oil, 2 tablespoons basil, garlic, pepper, and remaining ¼ teaspoon salt; set aside. Heat remaining 1 tablespoon oil in 12-inch nonstick skillet over medium heat until shimmering. Add prosciutto and cook, stirring frequently, until crispy, about 7 minutes. Using slotted spoon, transfer to paper towel–lined plate and set aside to cool.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Grease bottom of flat-bottomed, round, 2 ½-inch-wide dry measuring cup or drinking glass. Press cup firmly into center of each dough round until cup touches sheet to make indentation for filling. Divide reserved ricotta mixture evenly among each indentation (about 2 tablespoons each) then spread into smooth layer. Divide reserved zucchini evenly among each tartlet, shingling zucchini decoratively on top of ricotta.
Gently brush dough with egg wash and bake until light golden brown, 15 to 20 minutes, switching and rotating sheets halfway through baking. Transfer tarts to wire rack and let cool for 15 minutes. Sprinkle with reserved crispy prosciutto and remaining 2 tablespoons basil. Serve warm or at room temperature.