Slowly dissolve the sugar, milk, and butter until it's smooth. This can be done in a pot on the stove but it's also easy to do in the microwave and lowers the risk of burning. Put in a microwave-safe bowl and blast for 90 seconds at a time on a low or defrost setting. It'll take approximately 6 times in the microwave for it to melt into a smooth mixture.

Pour the liquid into a large pot on the stove. If you're using an electric stovetop you can use a trivet to stop the mixture burning.

Add the condensed milk and bring it up to a boil (stir continuously to stop it catching on the bottom of the pan)

Briskly simmer for about 20 minutes until the mixture darkens to a caramel colour, stirring continuously. Use a sugar thermometer to check temperature 120 degrees C...soft ball point

Take off the heat, quickly beat the mixture and pour it into a buttered swiss roll tin