Preheat oven to 425°. Place squash, flesh side up, in a 13- x 9-inch baking dish; drizzle with oil, and sprinkle with paprika and desired amount of salt and pepper. Turn squash, and place, flesh side down, in baking dish. Cover with heavy-duty aluminum foil. Bake 30 to 40 minutes or until tender. Cool in pan on a wire rack 5 minutes. Peel squash, and cut into large pieces.
Pulse squash, vinegar, and butter in a food processor 7 or 8 times or until smooth. Add salt and pepper to taste.
Spread 1 tsp. squash mixture onto each Crostini. Top each with a few pieces of shaved Parmesan and about 1/4 tsp. pecans.
Make-Ahead Tip: Chill squash mixture up to 2 days. Microwave, stirring every 30 seconds, until hot.