Pour ½ a tin of condensed milk in a medium-sized pot.
Use the empty tin and measure two tins of warm water into the pot. Stir to dissolve the condensed milk completely.
Place the milk mixture on medium heat and bring up to temperature, but don't let it come to a boil. Stir occasionally.
While the milk mixture is heating up, mix the custard powder with two tablespoons of cold water to form a slurry, making sure there are no lumps of powder.
When the milk mixture is steaming, use a ladle to mix a little of the hot milk into your cold custard powder mixture.
Once mixed, add custard powder mixture to steaming milk while stirring constantly. Once the mixture starts to thicken, reduce the heat and let it cook for 2 minutes to ensure the flour is cooked.
Pour into a jug and serve hot or cold over puddings, jelly or ice-cream.