Preheat oven to 180 degrees C. Grease and flour a ring tin. Sift together flour, baking powder, cinnamon, bicarb 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 tin. Bake for about 1 hour and 20 minutes, or until a skewer 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 icing sugar with 3 to 5 teaspoons orange juice to achieve drizzling consistency. Spoon over warm cake and sprinkle with orange zest if desired.