To make the caramelised breadcrumbs, heat oven to 180C/160C fan/gas 4. Tear half the bread into little pieces (it’s better doing this by hand than in a food processor. The largestpiece should be no bigger than a petit pois). Whizz the rest in a food processor into coarse crumbs. It’s best to have a mixture of textures.
In a frying pan, heat the butter until it melts, then continue to cook until it starts to brown a little. Remove from the heat and stir in the bread and sugar. Spread out on a baking sheet (it’s important that the mixture is not in clumps) and cook for 20 mins, tossing a few times during baking, until toasted. Leave to cool.
Heat the cream with 100g sugar and a pinch of salt in a heavy-bottomed saucepan until it comes to just under the boil. Take off the heat. Leave to sit for about 5 mins. Beat the egg yolks in a bowl with the remaining sugar and pour in the cream, stirring continuously. Wash out the saucepan, then pour the cream mixture into it.
Set over a low heat and cook, stirring, until the mixture thickens enough to coat the back of a spoon. It will take about 8-10 mins. Make sure the custard doesn’t boil or it will curdle. As soon as it’s thick enough, transfer to a cold bowl and stir in the buttermilk. Keep stirring the mixture at intervals as it cools. Cover the custard and put in the fridge to chill.
Churn the custard in an ice cream machine following the manufacturer’s instructions, adding the breadcrumbs when the ice cream is almost frozen. Transfer to a container, cover with cling film and keep in the freezer. If you don’t have an ice cream machine, pour the mixture into a shallow container and put in the freezer. Remove from the freezer every couple of hours and beat vigorously to break up the ice crystals, adding the breadcrumbs when the ice cream is almost frozen. You want to end up with a smooth, creamy mixture.