Peel the zest off the lemon in long strips, making sure not to get too much pith.
Pour the double cream into a medium sized saucepan and add the lemon peel.
Bring up to a gentle heat and then remove from the heat just before it reaches a boil.
Pour the cream and peel into a jug and leave to infuse at room temperature for at least 30 minutes. Strain out the lemon peel once infused and weigh out 300ml of the cream to use in this recipe.
Tip the caster sugar into a medium sized saucepan and heat on a gentle temperature as the sugar begins to melt.
Avoid moving the sugar around too much but you can encourage an even melting by gently shaking the actual saucepan around occasionally if you need to. Keep a careful eye on the sugar and do not leave at all.
As the sugar has melted it will turn a dark golden tan colour. Watch to make sure it doesn’t get too browned as this means it’s burning.
As soon as the sugar has melted and has turned that dark golden tan colour then pour in the strained cream and the butter. It will rise dramatically up the saucepan so stand back at first for safety.
Stir the cream and butter into the melted sugar and it will quickly form a thick silky caramel sauce.
Stir for a minute or two once the caramel has formed then you can turn off the heat and add the salt.
Pour the caramel into a jug and leave for 20 minutes for it to thicken to a pourable consistency. Stir briefly before using.