If you want to be uber fancy and impress your guests, you can dust your mousse with icing sugar before serving, or garnish the top with shavings of milk or white chocolate, but this totally optional.
Perhaps the easiest, most delicious and widely available vegan alternative which serves as a classic Christmas treat is chocolate ... as Sticky Toffee Pudding, Oat M!lk Hazelnut Latte, Panela Panache ...