Preheat your oven to 400 degrees F. Grease two 6-ounce ramekins with butter and place them on a rimmed foil-lined baking dish (to catch any spills).
In a medium bowl, whisk the egg whites until light and frothy. Add the salt, pepper, garlic powder, and parmesan, and whisk to incorporate.
Mix in the green onions with a spatula. Pour the mixture into a spouted measuring cup and from the measuring cup into the greased ramekins. The ramekins should be 3/4 full - if the mixture comes higher than that, use one extra ramekin to prevent spilling.
Place the ramekins on the foil-lined baking dish. Bake them until puffed, browned, and a knife inserted in center comes out clean, 20-25 minutes.