2) In a mixing bowl, whisk together the flour, baking powder, cinnamon and salt. Set the mixture aside. In a separate large mixing bowl, mash the sweet potato, leaving only a slight meaty texture. Whisk together the sweet potato and the eggs. Whisk in the warm water, then the sugar, then the oil followed by the vanilla essence. Stir in the dry ingredients until all combined then fold in sultanas and walnuts.
3) Spoon the batter into the prepared bundt pan. Bake until a toothpick inserted into the centre of the cake comes out clean, about 1 hour. Let the cake cool in the tin for 15 minutes and invert onto a serving platter or cake stand.
4) Meanwhile, for the glaze, melt the butter in a medium saucepan with the rum. Pull from the heat and whisk in the icing sugar until smooth. Pour the glaze over the cooled cake. Slice and serve.