If starting with raw sweet potato, peel and chop, then microwave on high till very soft. Mash well and let cool.
Preheat oven to 170 C / Gas 3. Generously butter a cake tin, about 33x23cm in size.
Add flour, cinnamon, ginger, baking powder, bicarb and salt into large bowl with 500g of the sweet potato mash. Add sugar and oil, beat until smooth. Add eggs 2 at a time, beating well after each, then add the vanilla. Pour into the prepared cake tin.
Bake about 1 hour 5 minutes, or until a skewer inserted in the centre comes out clean.
For the icing, stir the sugars, whipping cream and butter in saucepan over medium-low heat until butter melts and sugar dissolves. Increase heat to boil 3 minutes, stirring and swirling the pan. Remove from heat and stir in vanilla.
Whisk icing until smooth and lightened in colour, about 1 minute. Cool icing until icing falls in heavy ribbon from spoon, about 15 minutes.
Spoon icing over cooled cake. Let stand until icing is firm, at least 1 hour. (Can be prepared 1 day ahead.)