Infuse the fresh mint by pouring the milk and cream into two separate saucepans. Split the mint between the two and heat both until just below boiling point. Leave to cool for a couple of hours, then strain and discard the mint leaves from both saucepans. Measure out 250ml of double cream and 500ml whole milk as those are the quantities you will be working with. If there is any leftover you can save for another use or discard.
Fill your kitchen sink halfway up with water and pour in a lot of ice until freezing. Place a large mixing bowl into the water, so it comes halfway up the sides then pour half of the minted milk into the mixing bowl. Set a sieve over the top.
Spread the sugar in a large saucepan in an even layer. Heat it up until the edges begin to melt, fold it into the centre of the sugar carefully, stirring until it’s all dissolved. Carry on cooking until the caramel begins to smoke then remove from the heat immediately.
Add the butter and a pinch of salt until the butter has melted then stir in the cream. Don’t worry if the caramel seizes as it can melt again at the next stage.
Place back on the heat and stir until all the caramel has melted. Then stir in the second half of the milk gradually.
Whisk the yolks in a bowl, then whisk in some of the warm caramel so the eggs warm up, pour the eggs back into the caramel custard and heat. Stirring all the time until it begins to thicken.
Pour into the sieve set above the rest of the minted milk in the ice bath, add the vanilla extract and then whisk constantly until the temperature has cooled.
Pour the custard into a large jug, cover with cling film and place in the fridge overnight to chill.
The next day churn in your ice cream machine until the consistency of a thick milkshake. Decant into tubs and place in the freezer overnight before serving.