Using an electric mixer on high speed, beat eggs in a large bowl until light, airy, pale in color, and can nearly hold peaks, 6–8 minutes.
Reduce mixer speed to low. Gradually add flour and salt, beating well between additions, until no clumps of flour remain. With mixer speed on low, slowly pour in 1½ cups water and continue to beat until well incorporated. Cover and let dough rest at least 2 hours (this gives the flour time to hydrate and results in a smoother finished texture).
Heat a small nonstick skillet over medium and spray with nonstick spray. Pour 2 Tbsp. batter into skillet and cook, swirling skillet constantly to encourage crepe to form a nice, round shape, until set, 15–20 seconds. Tap out crepe onto a clean towel; gently flatten any creases. Repeat with remaining batter; if possible, do not to stack crepes.
Do Ahead: Dough can be made 1 day ahead; keep covered and chill.
Filling and Assembly
Preheat oven to 350°. Mix egg yolks, ricotta, mozzarella, and Parmesan in a large bowl; season with salt and pepper. Squeeze out excess liquid from spinach, if using, and add to egg mixture; toss to coat.
Spread 1 cup sauce in a 13x9” baking pan. Working one at a time, spoon 2 Tbsp. filling in center of crepe. Spread filling all the way to the ends. Roll up crepe, then flatten slightly with your hands. Transfer seam side down to prepared dish. Working in batches if needed, repeat with remaining crepes and filling, leaving a small gap in between each. Top with sauce and cover with foil. Bake manicotti until cheese is melted and sauce is bubbling, 35–45 minutes. Let manicotti cool slightly in pan to set, about 15 minutes. Top with Parmesan and pepper before serving.
Do Ahead: Filling can be made 1 day ahead; cover and chill. Manicotti can be made 1 month ahead; transfer to airtight containers or freezer bags and freeze.