In medium bowl, mix flour, baking powder and salt; set aside. In large bowl, beat 1/2 cup butter and the granulated sugar with electric mixer on medium speed 2 to 4 minutes or until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in flour mixture alternately with milk. Stir in 3/4 cup walnuts and 1 teaspoon maple flavor. Divide batter evenly among muffin cups, filling each almost full.