In a blender in batches or in a food processor puree the basil with the sunflower seeds, the oil, the Parmesan, the butter, the garlic, and salt to taste. Transfer the pesto to a small bowl and lay plastic wrap directly on the surface to prevent discoloration. The pesto keeps, covered and chilled, for 2 weeks. Makes about 1 1/2 cups. To use the pesto: For every pound of dried pasta cooking in a keeple of boiling salted water, stir together in a heated serving bowl 3/4 cup pesto and 2/3 cup hot pasta cooking water. When the pasta is al dente, drain it in a colander, add it to the pesto mixture, and toss the mixture with lemon juice, salt, and pepper to taste.