Place the caster sugar, coffee, liqueur and double cream in a large bowl and beat with a hand-held electric whisk until thickened and the whisk leaves a trail when lifted above the mixture. Pour into 4 tall glasses and chill while you make the topping.
To make the topping, place the whipping cream, vanilla sugar, whiskey, if using, and measurement water in a bowl and beat with a hand-held electric whisk until thick and frothy.
Spoon the cappuccino topping over the syllabubs and dust with cocoa powder. Serve immediately or chill until ready to serve.