Beat egg yolks, 4 tablespoons of the white sugar and vanilla in a mixing bowl with an electric mixer until thick and creamy.
Stir cream in a medium saucepan over low heat until it almost comes to boil. Remove from the heat and add cream to egg yolk mixture. Beat until combined.
Pour cream mixture into the top of a double boiler or a medium bowl set over a medium saucepan of simmering water. Stir until mixture lightly coats the back of a spoon, then pour mixture into a shallow 1 litre heatproof dish.
Bake for 30 minutes or until just set. Remove from oven and cool to room temperature, then refrigerate for 3 hours or overnight.
Before serving combine remaining white sugar with the brown sugar in a small bowl. Sift sugar mixture evenly over custard. Cook under a preheated griller for 2 minutes or until sugar caramelises. Remove from heat and cool 10 minutes. Serve.