Let egg whites stand at room temperature for 30 minutes.
In a large bowl, combine the flour, walnuts, 1/2 cup sugar, brown sugar, baking powder and salt. In another large bowl, whisk the egg yolks, water, oil and maple flavoring; add to dry ingredients and beat until well blended. In a small bowl, beat egg whites, cream of tartar and remaining sugar on medium speed until stiff peaks form; fold into batter.
Gently spoon into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets. Bake on the lowest oven rack at 325° for 60-70 minutes or until top springs back when lightly touched.
Immediately invert the pan onto a wire rack; cool completely. Carefully run a knife around edges of pan and center tube to loosen; remove cake.
For glaze, heat butter in a small saucepan over medium heat until golden brown. Remove from the heat; stir in confectioners' sugar, vanilla and enough water to achieve desired consistency. Drizzle over cake.