Let egg whites stand at room temperature for 30 minutes. Sift together flour and brown sugar twice; set aside. In a large bowl, beat the egg whites, cream of tartar and salt on medium speed until soft peaks form. Add vanilla and maple flavoring. Gradually beat in sugar, 2 tablespoons at a time, on high until stiff glossy peaks form and sugar is dissolved. Gradually fold in flour mixture a fourth at a time.
Spoon half the batter into an ungreased 10-in. tube pan; sprinkle with nuts. Cut through batter with knife; spoon in remaining batter. Bake on the lowest rack at 375° for 35-40 minutes or until lightly browned and cracks feel dry. Immediately invert baking pan; cool completely, about 1 hour.
Run a knife around sides and center tube of pan. Invert cake onto a serving plate.