Preparation Heat 2 tablespoons oil in a large skillet, preferably nonstick, over medium-high. Add leek, season with kosher salt and pepper, and cook, stirring occasionally, until leek is softened and starting to brown, about 4 minutes. Add squash and season again. Cook, stirring often, until squash is cooked through and softened, about 4 minutes. Transfer vegetables to a plate and let cool. Wipe out skillet and reserve. Meanwhile, whisk egg, chickpea flour, baking powder, 1 tablespoon oil, 1/2 teaspoon kosher salt, and 1/2 cup water in a medium bowl; season with pepper and let sit 5 minutes for flour to hydrate. Stir vegetables into batter just to coat. Heat 1 1/2 tablespoons oil in reserved skillet over medium-high. Add batter by the 1/4-cupful to make 4 pancakes, gently flattening to about 1/4" thick. Batter should spread easily—if it doesn't, thin with a little water. Cook until bottoms are lightly browned and bubbles form on top, about 4 minutes. Use a spatula to carefully flip pancakes over and cook until browned and cooked through, about 2 minutes longer. Transfer to a plate and tent with a sheet of foil to keep warm. Repeat with another 1 1/2 tablespoons oil and remaining batter. Serve pancakes topped with yogurt, parsley, sea salt, and pepper. Do ahead: Leek and squash can be cooked 2 days ahead; cover and chill. Batter can be made 1 day ahead; cover and chill.