Place the Muscovado sugar and butter in a pan and heat until melted.
Allow to bubble for 1 minute.
Add the warm milk, stirring until gently heated through.
Beat the eggs and sugar until thoroughly mixed then stir in the warm milk mixture and vanilla paste.
Use a sieve to strain the mixture back into the pan.
Stir over a low heat until the custard thickens slightly (it mustn't boil or simmer or will curdle). It should just coat the back of a spoon.
Cool the custard by pouring it into a bowl and place it in a larger bowl, a third full of iced water – this takes about 20 minutes.
Meanwhile whisk the double cream lightly (until forms floppy peaks, you don't want stiff cream). Place in the fridge until custard has cooled.
Once the custard has cooled mix in the cream and salt flakes and put in the fridge for at least 10 minutes.
Chop up the Crunchie bars into small nuggets (if you do this any earlier they just start to melt).
Pour the ice cream mixture into an ice cream maker, switch on and then pour the Crunchie pieces down the shoot in order to mix them in.
Once the ice cream is ready, spoon it into a suitable container (if you can wait) and allow it to freeze fully for a few hours before serving.