Preheat oven to 425 degrees F. Line two large baking sheets with parchment paper.
In medium pot, melt butter over medium heat. Once butter is foaming, add garlic and sauté 30 seconds, or until garlic is fragrant. Add milk, water and salt. Bring to a light boil. Remove from heat and add flour at once, vigorously stirring with a wooden spoon, until it forms a ball of dough. The dough might form a light crust on the bottom, and that’s fine! Keep stirring until it’s a smooth ball.
Transfer dough to a large bowl and let cool a few minutes. Add eggs, one at a time, stirring again with a strong arm until the egg is completely incorporated into the dough. It might look lumpy at first, but the more you stir, the glossier the dough will get.
Add 1/4 cup gruyere, parmesan and chives; stir until dough is shiny.
Drop rounded tablespoons of dough onto prepared pans. Sprinkle the remaining gruyere over each puff.
Place pans in the oven and immediately reduce heat to 375?F. Bake 12 minutes, then flip the pans (if they’re on upper and lower racks), and bake another 12 minutes, or until puffed and golden brown.