Put the cream, chocolate, peanut butter and syrup in a non-stick pan and let it heat, stirring constantly, until everything has melted and the ingredients have combined. It is important to use a pan, as the sauce cannot `caramelize` as well in a double boiler.
It should now be a slightly viscous, shiny brown mass.
Set the pan aside and put 2 large scoops of vanilla ice cream in each of 4 sundaes. Spread the slightly warm fudge sauce over it and then sprinkle each cup with a tablespoon of hazelnut brittle. Serve immediately.