Roast the squash cut side up on a baking sheet in preheated oven until the flesh is soft, about 50 minutes. Remove squash from oven and cool, then scoop out the flesh into a mixing bowl, or bowl of a food processor.
Meanwhile, prepare the topping by mixing 2 1/2 tablespoons butter in a bowl with the brown sugar and flour until crumbly. Stir in the pecans. Set aside until needed.
Lower oven temperature to 350 degrees F (175 degrees C). Lightly grease 11×7 inch baking dish.
Beat or process the squash until smooth. Add the eggs, white sugar, half and half, vanilla, salt, and 2 1/2 tablespoons butter; process until well blended. Spoon the mixture into the prepared baking dish. Sprinkle the brown sugar topping mixture over the squash.
Bake squash in preheated oven until topping is lightly brown, about 40 minutes