Place the butter, cream and chocolate into a small saucepan over a low heat. Stir gently and constantly until the chocolate has melted. Take off the heat and stir in the golden syrup and milk.
Remove the cake from the fridge, still on the rack, and set it over a baking tray. Pour the glaze all over the cake, coaxing it with a spoon if needed, so that it covers everything.
Leave to cool for a few minutes then use a metal spatula and your hands to transfer the cake to a serving platter. Decorate with shaved white chocolate and cocoa nibs. Chill for at least 30 minutes and up to 2 days before serving