1. Preheat oven to 425°. Heat butter and 1 cup water in a medium saucepan over medium heat until butter is melted and mixture comes to a simmer.
2. Turn heat to low, add flour, and stir vigorously until mixture forms a ball that pulls away from pan sides, about 1 minute. Remove pan from heat.
3. Add eggs, one at a time, stirring vigorously after each. Dough will separate after each addition; keep stirring until it forms a smooth paste.
4. Stir in gruyère cheese, parmesan cheese, Dijon mustard, and freshly ground black pepper.
5. Drop mixture in heaping tablespoonfuls onto 2 buttered baking sheets. Bake until puffed and brown, about 30 minutes. Cut slits in sides of puffs, return to oven, and lower heat to 350°. Bake 10 minutes.