4 tablespoons unsalted butter, plus more for ramekins
2 tablespoons finely grated Parmesan cheese (1/2 ounce)
1/4 cup all-purpose flour
1 cup whole milk
1 cup grated Gruyere cheese (2 ounces)
2 tablespoons minced chives, plus 1 tablespoon 1/2-inch pieces
Coarse salt and freshly ground pepper
3 large eggs, room temperature, whites and yolks separated
1. Preheat oven to 400 degrees. Butter two 2-cup ramekins, coat with Parmesan, and refrigerate. Melt butter in a saucepan over medium heat. Add flour; stir 1 minute. Add milk, whisking until thickened, 2 minutes. Transfer to a large bowl. Whisk in Gruyere, minced chives, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Whisk in egg yolks.
2. Whisk egg whites until stiff peaks form. Fold into cheese mixture in thirds. Divide between ramekins. Bake on a baking sheet 15 minutes. Reduce heat to 375 degrees; bake until puffed, about 15 minutes.