Beat butter in second bowl until smooth and creamy. Beat in granulated sugar and maple syrup. Add eggs, one at a time, beating after each. Drain potatoes; puree in food processor or mash with fork. Add potatoes and 1/2 teaspoon maple extract to butter mixture; beat until smooth. On low speed, beat in flour mixture just until incorporated. Fold in walnuts. Transfer batter to pans, dividing evenly.