Preheat oven to 425°F. Line 2 baking sheets with parchment paper and set aside.
Add the butter, water, sugar, and salt to a medium saucepan set over medium heat. Stir until the salt and sugar are melted. Bring the mixture to a boil.
¼ cup salted butter, ½ cup water, 1 tablespoon granulated sugar, ½ teaspoon sea salt
Add in the flour and stir until it forms a ball. Continue to stir until the ball becomes smooth and there is a film on the bottom of the pan.
1 cup all-purpose flour
Remove from the heat and spread the mixture out on the bottom of the pan. Let it cool for 1-2 minutes.
Add the eggs one at a time, stirring until fully incorporated before adding the next egg. Stir until the dough is smooth. Before the last egg is fully combined, add in both cheeses and stir.
4 large eggs, 1 cup finely grated cheddar cheese, ½ cup finely grated gruyere cheese
Using a 2-ounce cookie scoop, scoop the dough onto the prepared baking pans, leaving 2 inches between each puff.
Whisk together the egg and water. Gently brush each puff and sprinkle with Parmesan cheese.
1 large egg, ¼ cup freshly grated Parmesan cheese, 1 tablespoon water
Bake for 10 minutes.
Lower the oven to 350°F bake for 15-20 more minutes until golden brown.