Put the sugar in a large saucepan. Turn the heat to a medium temperature and carefully melt the sugar. Gently tilt the pan as the sugar melts, rather than stirring, as it will adhere to a spoon or spatula.

When the sugar is melted, add the butter and stir into the sugar. It will bubble, so take care. Next, add the milk a little at a time, stirring continuously.

Finally, add the can of condensed milk and stir well until it is fully incorporated. Keep stirring as the dulce de leche bubbles and thickens over a medium heat. It will need a further 5-10 mins of cooking. It will turn a darker shade of brown and take on a slight orange-red hue as it darkens. It should be thick enough to leave a clear channel in the pan when a spoon is run through and then add a pinch of salt. It will firm up as it cools. The dulce de leche can be stored in a jar in the fridge and will keep for up to a month. If not storing in a jar, ensure it's covered so a skin doesn’t form.