1Heat oven to 350°F. Spray bottom of 13x9-inch pan with cooking spray.
2In large bowl, beat 3/4 cup butter and the granulated sugar with electric mixer on medium speed until light and fluffy. Beat in eggs until fluffy. Beat in baking powder, baking soda, salt, ginger, cinnamon and nutmeg. Add flour alternately with buttermilk, scraping side of bowl. Beat 1 minute. Stir in squash and 1/2 cup walnuts. Spread in pan.
3Bake 30 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely.
4In medium bowl, beat 1/2 cup butter and the cream cheese with electric mixer on medium speed until light and fluffy. Add powdered sugar, 2 tablespoons milk and the maple flavor. Beat until smooth and creamy, adding additional milk if needed. Frost cake. Sprinkle with 1/2 cup walnuts. Cover and refrigerate.