Preheat oven to 180 C / Gas 4. Grease and flour a 26cm cake ring tin. Sift together flour, baking powder, cinnamon, bicarbonate of soda, nutmeg and salt. Set aside.
In large mixing bowl, beat 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 cake tin. Bake for about 1 hour and 20 minutes, or until a wooden skewer inserted into cake comes out clean. Cool cake for 20 minutes in the tin, then turn out onto serving plate.
To make the glaze: Combine 125g icing sugar with 3 to 5 teaspoons orange juice in a small bowl, mix until you achieve drizzling consistency. Spoon over warm cake and sprinkle with orange zest if desired.