Preheat the oven to 400 degrees F (200 degrees C).

Roast squash cut-side up on a baking sheet in the preheated oven until flesh is soft, about 50 minutes. Remove squash from the oven and cool, then scoop flesh into a mixing bowl or the bowl of a food processor.

Meanwhile, prepare topping by mixing 2 1/2 tablespoons butter in a medium bowl with brown sugar and flour until crumbly. Stir in pecans. Set aside until needed.

Lower oven temperature to 350 degrees F (175 degrees C). Lightly grease an 11x7-inch baking dish.

Beat or process squash flesh until smooth. Add eggs, white sugar, half-and-half, remaining 2 1/2 tablespoons butter, vanilla, and salt; process until well-blended. Spoon mixture into the prepared baking dish. Sprinkle brown sugar topping mixture over squash.

Bake in the preheated oven until topping is lightly brown, about 40 minutes.