Place the potato in a large Pyrex bowl. Add in half the butter and all the water. Cover with cling film.
Microwave on HIGH (900 watt) for 12-13 minutes, until the sweet potato is tender.
Drain in a sieve, reserving the drained liquid. Place in a food processor with the crumbled Knorr Reduced Salt Vegetable Stock Cube.
Blend into a smooth puree, adding a little of the reserved cooking liquid or a little boiling water if needed.
Recipe courtesy of Knorr