Place the cauliflower florets in a large pot and cover with water so they are submerged. Bring to a boil over high heat.
Reduce heat to low, cover, and simmer until very soft, about 22 minutes. Drain in a colander and allow to cool until safe to handle.
Working in batches, transfer 1/3 to 1/2 of the cooled cauliflower to a clean kitchen towel or cheesecloth, gather the cloth, and squeeze out as much water as possible. Add the drained cauliflower to a large bowl and repeat with the remaining cauliflower.
Add the flour and kosher salt to the drained cauliflower. Mix first with a fork, then use your hands to fold and squeeze until a cohesive dough forms; avoid overworking.
Lightly dust a work surface with flour. Divide the dough into 8 equal pieces (about 2½ ounces each). Roll each piece into a ½-inch-thick rope about 10 inches long.
Cut each rope into roughly 13 equal pieces to form gnocchi (about 108 pieces total). If the dough gets sticky, dust hands with flour as needed.
Heat a large skillet over medium and mist liberally with olive oil spray. Working in two batches, add half the gnocchi to the pan without overcrowding.
Let the gnocchi cook undisturbed for 2 minutes, then gently flip with a spatula or tongs. Sprinkle with a little salt and cook 2–4 minutes more until cooked through and lightly browned. Transfer to a plate and repeat with remaining gnocchi.
Return all cooked gnocchi to the skillet, pour the marinara sauce over them, stir gently to combine, and heat until warmed through. Serve topped with grated Parmesan and basil if using.
