Rub a 2-quart casserole dish with butter.
Grind the toasted sunflower seeds in a blender or food processor. Set aside.
Bring a pot of salted water to a boil. Preheat oven to 350° F.
In a saucepan, melt the first 2 tablespoons of butter.
Stir in the rice flour. Over medium flame, cook and stir, letting the butter and flour bubble a minute. Gradually whisk in the vegetable broth, whisking constantly until the sauce thickens a bit, not quite the thickness of heavy cream.
Using a wooden spoon, stir in the grated cheese about ¼ at a time. Stir in the cayenne, a small pinch or larger pinch—to your taste. Taste for salt, adding if needed. Set aside while you cook the pasta.
Stir the pasta into the boiling water. Cook until the pasta is tender, but with a bite, al dente. (Be sure not to overcook as the pasta will plump a bit more with the sauce when it bakes.) Drain. Stir into the cheese sauce. Pour into the prepared pan.
Spread the ground sunflower seeds on top. Dot with the remaining butter. Bake, uncovered, about 20 minutes, until the cheese is bubbling through the crust.