In a large bowl, combine the squash, 1/4 cup cracker crumbs, 2 tablespoons butter, egg, brown sugar, mustard, salt and pepper.
Pour into a greased 2-1/2-cup baking dish. Combine remaining crumbs and butter; sprinkle over top. Bake, uncovered, at 350° for 20 minutes or until a knife inserted in the center comes out clean.