Pre-heat oven to 325° (160°C). Move oven rack to the lowest position.
In large bowl, whisk together flour, sugar, baking powder, cinnamon and salt. Add in water, oil, vanilla, maple and egg yolks. Using an electric mixer, beat until blended. Stir in walnuts.
In separate large bowl, using clean beaters, beat egg whites and cream of tartar on high speed until stiff peaks form, about 2 minutes. Fold one-quarter of the egg white mixture into yolk mixture. Fold in remaining whites until no streaks remain. Scrape into ungreased 10-inch (25 cm) tube pan. Run spatula through the batter to eliminate any air bubbles; smooth top.
Bake until top springs back when lightly touched, about 75 minutes. Turn pan upside down and let hang on legs attached to pan or on a bottle, until completely cooled, about 2 hours. Run knife around inside of pan and tube. Turn out, bottom side up onto a cake plate.
Fluffy Maple Frosting: In heatproof bowl, combine egg whites, brown sugar, water and corn syrup. Using an electric mixer, beat until well blended. Set bowl over rapidly boiling water. Beat on high speed and cook until mixture forms stiff peaks, about 5 to 7 minutes. Remove from heat and add maple extract. Beat for about 1 minute or until spreading consistency. Frost cooled cake. Garnish with chopped walnuts, if desired.
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