125 grams butter
1 can (395 grams) sweetened condensed milk
1 cup sugar
2 tablespoons golden syrup
Melt butter and sugar over a low heat. Add condensed milk and golden syrup. Stir continuously until mixture reaches soft ball stage* and is golden brown. Remove from heat. Pour mixture into a greased 15 cm square tin. Cool until set and cut into squares.
* I don't have a candy thermometer and find it fiddly testing the mixture in cold water. If you can complete a figure eight with your spoon while stirring the caramel (i.e. the start of the eight is still visible when you finish drawing it) that's also a reasonable approximation of 'soft ball stage'.
As a final step, grind some sea salt over the caramels