Put the custard powder in a non-stick pan and stir in the sugar and 150ml of the cream until smooth. Gradually stir in the rest of the cream until smooth then gently heat until simmering, stirring all the time. It will thicken quite quickly so keep an eye on it and keep stirring. As soon as it thickens, beat well then take off the heat and tip into a bowl. Cover the surface with a sheet of wax paper or baking paper to stop a skin forming, and cool.