Line the base and sides of a 20cm square cake tin with baking paper. Put sugar and evaporated milk in a saucepan over a low heat and stir until sugar dissolves. Bring to the boil, then reduce to a simmer, being careful mixture doesn’t boil over. Stir often for 15-20 minutes until mixture reaches 110-115°C on a sugar thermometer, or when a teaspoon of mixture dropped into cold water forms a soft ball. Remove from heat. Allow to stand for 20 minutes to cool.