Bring the water and sugar to the boil in a wide pan. Add the rhubarb, cover and simmer over a low heat for approx. 10 mins., leave to cool.
Place the butter in a bowl and beat in the sugar, vanilla and salt. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Stir in the sour cream. Mix the flour and baking powder, stir into the mixture.
Remove the rhubarb from the poaching liquid, drain, retain the liquid and set aside. Layer the cake batter and rhubarb in the prepared tin, starting and ending with a layer of cake batter.
Approx. 1 hr. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tin and leave to cool on a rack. Mix the icing sugar with 2 tbsp of the reserved rhubarb liquid. Pour the glaze over the cake, leave to dry.