For the butterscotch cream, melt the caster sugar in a small saucepan until it starts to caramelise, then leave a little longer until it starts to turn a ruby red colour. Watch it carefully – you don’t want it to burn. Remove from the heat, and whisk in the brown sugar and butter, followed by 100ml of the cream in a thin stream, while still whisking. Return to the heat and bring to a foamy boil for about 5 minutes, whisking occasionally until all the sugar has dissolved and there are no lumps. Remove from the heat and allow to cool completely.