Bring a large pot of salted water to a boil. Add cauliflower florets and cook until very tender when pierced with a knife, about 10-12 minutes. Drain thoroughly and let steam escape for 5 minutes - excess moisture is your enemy here.

Transfer cauliflower to a mixing bowl and mash with a potato masher until mostly smooth with some small chunks remaining for texture. Let cool 10 minutes so the eggs don't scramble when added.

Stir 3/4 cup cheddar, all the Parmesan, eggs, panko, green onions, garlic powder, cayenne, salt, and pepper into the cooled cauliflower. Mix until well combined. The mixture should hold together when squeezed - if too wet, add 2 more tablespoons panko.

Scoop about 1/3 cup mixture and form into a patty about 3 inches wide and 3/4 inch thick. Press both sides lightly into extra panko for a crispier crust. Place on a plate and repeat with remaining mixture to make 8 patties.

Heat 2 tablespoons oil in a large nonstick skillet over medium heat. When shimmering, add 4 patties without crowding. Cook until deeply golden brown and crisp on the bottom, 4-5 minutes. Flip gently and cook another 3-4 minutes. Transfer to a plate and repeat with remaining oil and patties.

Sprinkle remaining 1/4 cup cheddar over the hot cakes and tent with foil for 2 minutes to melt. Serve warm with sour cream on the side if desired.