Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 10-inch tube pan. Sift together flour, baking powder, cinnamon, baking soda, nutmeg and salt. Set aside.
In large mixing bowl, cream butter and sugar until light and fluffy. Add mashed sweet potatoes and vanilla. Beat until well blended. Add eggs, one at a time (the batter will look curdled). Add flour mixture to potato mixture. Beat on low until combined.
Pour batter into 10 inch tube pan. Bake at 350 degrees F (175 degrees C) for about 1 hour and 20 minutes, or until a wooden toothpick inserted into cake comes out clean. Cool cake for 20 minutes in the pan, then invert onto serving plate.
To make the glaze:in a small bowl, combine confectioners sugar with 3 to 5 teaspoons orange juice to achieve drizzling consistency. Spoon over warm cake and sprinkle with orange zest if desired.
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