Put the butter, sugar, salt and 2 tbsp water in a medium saucepan and simmer over a medium to low heat for 8–10 minutes, stirring occasionally, until the mixture changes colour from pale straw to golden, and appears condensed.
2. Finish and cool
Remove the pan from the heat and stir in the cream; it will splutter. Pour the caramel into a large bowl, cover and leave to cool for 30 minutes.
3. Make the custard
Bring the milk to the boil in a small saucepan. Meanwhile, beat the egg yolks and sugar together in a large bowl, then whisk in the hot milk. The mixture should thicken slightly into a thin pouring custard.
4. Strain and chill the mixture
Sieve the custard onto the caramel and whisk to blend the two together. Cover the bowl with clingfilm and leave to cool: chilling the mixture before freezing it takes some of the strain off the ice-cream machine.
5. Churn the ice cream
Freeze the mixture according to the instructions for your ice-cream maker, then transfer it to a plastic container, seal and freeze. If the mixture has frozen solid, take it out of the freezer 20 minutes before serving to allow it to soften