Spinach and Gruyere Souffle

Name: Spinach and Gruyere Souffle
Ingredients: 2 tablespoons unsalted butter, room temperature, plus more for baking dish; ⅓ cup plain dried breadcrumbs; 5 cups (5 ounces) packed spinach, trimmed and washed; 2 tablespoons all-purpose flour; ¾ cup plus 2 tablespoons whole milk; ½ cup grated Gruyere cheese; Coarse salt and ground pepper; 2 large eggs, separated, plus 2 large egg whites