Place 1 in. of water in a saucepan; add squash. Bring to a boil. Reduce heat; cover and simmer for 5 minutes or until tender. Drain and mash; stir in the butter and egg. In a bowl, combine the flour, sugar, baking powder and salt. Stir in the squash mixture just until moistened.
Fill greased muffin cups three-fourths full. Bake 375° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from pan to a wire rack.
