Lightly grease a large ovenproof dish with butter. Butter the bread and stack into the oven dish. Sprinkle with the sultanas. Blend together the eggs, sugar, lemon zest, milk and vanilla essence until smooth. Pour over the bread and leave to soak for 5 minutes. Sprinkle with the brown sugar and bake at 160 C for 45 minutes until golden and risen. Place the sugar and water into a saucepan and heat gently until the sugar dissolves. Beat this syrup into the whisky and eggs. Transfer to a clean saucepan, add the butter, and cook over a low heat, stirring continuously until the sauce thickens. Stir in the cream and serve.