1. Preheat oven to 350°.
2. Combine first 3 ingredients in a medium bowl; beat with a mixer at medium speed. Add eggs, 1 at a time, beating well after each addition. Stir in vanilla. Increase speed to high; beat for 1 minute.
3. Weigh flour. Combine flour and next 3 ingredients (through cinnamon), stirring well. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture, beating until just combined. Stir in 1/3 cup walnuts.
4. Place 12 muffin cup liners in muffin cups; divide batter evenly among cups. Bake at 350° for 19 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. (Cupcakes will look slightly pale.) Cool in pan 5 minutes. Remove from pan; cool on wire rack.
5. Place cream of tartar and egg whites in a large bowl; beat with a mixer at high speed until soft peaks form. Combine maple sugar, 1/4 cup water, and dash of salt in a small saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 238°. Gradually pour hot sugar syrup into egg white mixture, beating until stiff peaks form. Spread about 3 rounded tablespoonfuls frosting over each cupcake. Sprinkle with remaining 2 tablespoons nuts.