1Heat oven to 350°F. Place mini paper baking cup in each of 36 mini muffin cups.
2In 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.
3Bake 17 to 18 minutes or until toothpick inserted in center comes out clean. Remove from pans to cooling racks; cool completely, about 15 minutes.
4In medium bowl, beat cream cheese and 1/4 cup butter until blended. Gradually add powdered sugar and 1/2 teaspoon maple flavor, beating until smooth. Frost cupcakes. Garnish with turbinado sugar and walnuts.