Melt the butter in a medium heavy-based saucepan. Add the brown sugar and cook for 2-3 minutes, stirring occasionally, until the sugar has dissolved. Add the cream and bring to a simmer, stirring until you have a nice thick butterscotch sauce.
Meanwhile, place the cornflour in a bowl with the salt and slate with a couple of tablespoons of the milk until smooth, then whisk into the rest of the milk in a large bowl. Whisk in the egg yolks with the vanilla extract and then gradually add the hot butterscotch sauce, whisking constantly.
Wipe out the saucepan. Strain the mixture back through a fine-mesh sieve back into the saucepan and cook over a medium heat for 5-6 minutes, stirring regularly, until the butterscotch custard starts to thicken.
Remove from the heat. Pour into pretty glasses or ramekins and chill for at least an hour or up to 3 days.
To make the caramel sauce, place the butter, sugar, and golden syrup into a saucepan and bring to a gentle boil simmer until the sugar has dissolved. Add the cream, vanilla and salt and whisk together; simmer for 3 minutes until the sauce is thick and sticky. Use right away or transfer to a jar and refrigerate when cool to use as needed. The sauce will keep for up to 7 days refrigerated.
To serve, drizzle the puddings with caramel sauce and top with a dollop of crème fraiche and a final drizzle of sauce.