Start by preheating your oven to 350 degrees Fahrenheit. Lightly grease a 2 quart baking dish with butter or a light coating of oil to ensure the casserole does not stick to the sides during baking.
If you are starting with fresh squash, ensure it is roasted or boiled until completely tender. Place the cooked squash in a large mixing bowl and mash it until it is smooth. You can use a potato masher or an electric mixer for a fluffier consistency.
Add the melted butter, granulated sugar, beaten eggs, milk, alcohol-free vanilla, cinnamon, nutmeg, and salt to the mashed squash. Stir everything together until the mixture is well combined and creamy. Pour the squash mixture into your prepared baking dish and smooth the top with a spatula.
In a separate small bowl, combine the chopped pecans, brown sugar, and all-purpose flour. Use a fork or your fingers to work the cold, cubed butter into the mixture until it looks like coarse crumbs. Sprinkle this topping evenly over the squash base.
Place the dish in the oven and bake for 40 to 45 minutes. The casserole is done when the topping is golden brown and the center is set. Remove it from the oven and let it sit for a few minutes before serving to allow the flavors to settle.
