Put the milk and sugar in a pot, and warm over medium heat until it begins to bubble.
Bring the heat down to low as soon as the milk starts to boil, and reduce while mixing with a wooden spatula (approximately 15-20 minutes).
Reduce it to half, and remove from the heat when you can see the bottom of the pot when you mix. It thickens when it cools, so it's okay if it's a little bit runny.
Place the pot which has been removed from the heat into ice water, and cool the mixture as you mix. Then it's complete.
Story Behind this Recipe
Condensed milk is expensive if you buy it, and it gets used up in the blink of an eye, so I decided to make it myself. There is absolutely no difference between using discounted milk that is close to its expiration date and high quality milk, so it's a great deal. I use a 1 L container of milk when making this, and store it in the freezer.
Be sure not to let it burn when mixing and reducing the milk. If you use white sugar, it will be slightly cream-coloured, but if you use raw sugar or brown sugar, it will have a slight amber colour. Choose whichever you like. You can also use soy milk, by the way.