Preheat oven to 400°F with a rack in middle position. Line a large rimmed baking sheet with foil. Remove and discard leaves from cauliflower. Slice off the bottom of the core so the cauliflower will sit flat on the prepared baking sheet. Brush 1 tablespoon oil all over the cauliflower. Sprinkle with ¼ teaspoon each salt and pepper.
Roast until deeply browned and tender, rotating the pan front to back halfway through, 50 to 60 minutes. Let rest until cool enough to handle, about 15 minutes. Increase oven temperature to broil.
Meanwhile, heat the remaining 1 tablespoon oil in a small skillet over medium heat. Add 2 tablespoons panko; cook, stirring often, until lightly browned, 1 to 2 minutes. Stir in ½ teaspoon Italian seasoning. Let cool completely, about 10 minutes.
When the cauliflower is cool enough to handle, separate the areas between the florets, being careful to keep the head intact; stuff evenly with ¾ cup cheese. Stir 1 tablespoon softened butter into the panko mixture until combined; spread over the cauliflower. Broil until the cheese is melted and the panko is browned on top, 3 to 4 minutes. Sprinkle with the remaining ¼ teaspoon salt. Sprinkle with 1 tablespoon basil and serve with ¾ cup warmed marinara.
