Cream butter and sugar; beat in maple syrup. Add vanilla and eggs, beating until mixture is smooth. Sift together the flour, baking powder, and salt. Add flour mixture to first mixture alternately with the milk, beating well after each addition. Stir in walnuts. Pour into 2 greased and floured 8-inch round cake pans. Bake at 375° for 30 to 35 minutes, or until a wooden pick or cake tester inserted in center comes out clean.
In a saucepan, cook 1 cup maple syrup until a soft ball forms when a small amount is dropped in cold water.
Meanwhile, beat egg whites until stiff peaks are formed, but mixture is not dry. Slowly drizzle the syrup into the beaten egg whites, beating all the while. Blend in 1 teaspoon vanilla and a pinch of salt. Beat until frosting is desired consistency.