Begin by making the caramel sauce. Combine sugar and water in a small saucepan. Stir over low heat until the sugar dissolves. Use a wet pastry brush to brush down the sides of the pan to dissolve any remaining sugar crystals. Increase heat to medium-high and bring to the boil. Reduce heat to medium and boil, uncovered, without stirring until the mixture turns a light golden colour. Remove from the heat and use a wooden spoon to stir in the brown sugar. Stir in the cream carefully as it may splutter until well combined. Return to medium heat and cook, stirring for 1 to 2 minutes, or until smooth. Pour into a clean jar or bottle and seal while hot. Allow to cool to room temperature and then store in the fridge.
Next make the meringue.
Beat the egg whites until stiff peaks form. Continue beating, adding the sugar a little at a time until dissolved. Sprinkle over the cocoa and vinegar and gently fold in.
On a baking tray lined with baking paper, spoon the meringue mix into a circular shape roughly the size of a dinner plate. Smooth the top so that there is a rim around the outer edge of the circle. Bake in a moderately slow oven 150°C for 45 minutes. Turn oven off, leave door ajar and cool meringue in oven. Remove and set aside.
Preheat oven to 160°C. Grease and line a 20cm x 30cm slice pan with baking paper, allowing the 2 long sides to overhang. Combine the chocolate and butter in a saucepan over low heat. Cook, stirring, for 5 mins or until chocolate melts and mixture is smooth.
Add the sugar and egg and stir to combine. Add the flour and cocoa, stirring until just combined. Add the sour cream, stirring, until just combined. Pour into the prepared pan. Bake for 30 mins or until a skewer inserted into the centre comes out clean. Set aside in the pan to cool completely. Turn the brownie onto a clean work surface. Using a dinner plate cut a large circle from the brownie. Cut a piece of baking paper 30cm long. Lightly grease the baking paper with cooking oil spray. Using the brownie disc as the base, wrap the baking paper, greased-side facing in, around the brownie making a tall collar. Secure with a staple.
Melt 150g of the white chocolate in a heatproof bowl over a saucepan of simmering water. Meanwhile, stir 1 tbs boiling water with ½ tsp of the gelatine in a small bowl until gelatine dissolves. Add to the melted chocolate and stir to combine. Stir in 4 tbsp caramel sauce. Separate one egg. Place the egg white in a medium bowl. Add egg yolk to chocolate mixture and stir to combine. Use an electric mixer to whisk ½ cup of cream until soft peaks form. Add to the chocolate mixture and stir to combine. Use a clean electric mixer to whisk the egg white until soft peaks form. Gently fold into the chocolate mixture until just combined. Spoon onto the brownie disc and smooth. Place in the fridge for 20 mins to set.
WHITE CHOCOLATE MOUSSE
Melt white chocolate in a heatproof bowl over a saucepan of simmering water. Stir 1 tbsp boiling water with 1/2 tsp gelatine in a small bowl until gelatine dissolves. Add to the melted chocolate and stir to combine. Separate egg. Place the egg white in a medium bowl. Add egg yolk to chocolate mixture and stir to combine. Use an electric mixer to whisk 1/2 cup cream until soft peaks form. Add to the chocolate mixture and stir to combine. Use a clean electric mixer to whisk the egg white until soft peaks form. Gently fold into the chocolate mixture until just combined. Spoon half the mixture onto the caramel layer and smooth. Reserve other half of mixture for later. Place in the fridge for 20 mins to set.
Place the meringue on top of the white chocolate mousse layer and return to fridge while you make the milk chocolate mousse.
MILK CHOCOLATE MOUSSE
Repeat the steps for the white chocolate mousse but using milk chocolate. Spoon over the meringue layer and return to the fridge for 20 mins to set.
Add the remainder of the white chocolate mousse onto milk chocolate layer. Place in fridge for 20 minutes to set.
DARK CHOCOLATE MOUSSE
To make the final layer of mousse, repeat the steps for white chocolate mousse but using dark chocolate. Pour over the white chocolate mousse layer and place in the fridge for 6 hours or until completely set.
Remove the paper collar from the cake. Serve with the caramel sauce.