Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a 20cm spring form tin with baking parchment and oil the sides with vegetable oil.
In a large bowl, whisk the egg white until foamy peaks form and then whisk in the sugar a little at a time to make a thick, mixture.
Sieve over the cocoa and sprinkle with the vinegar, then whisk again to combine everything.
Spread as evenly as you can over the bottom of the prepared cake tin and bake for 15-20 minutes.
Leave to cool.
Bring a saucepan of water to the boil, reduce the heat to a simmer and melt the chocolate with the rum in a bowl placed over the saucepan. Remove and let rest for 5 minutes.
Whisk the cream until it thickens slightly -like thick pouring custard.
Pour into the chocolate mixture, beating gently until everything is combined.
Pour into the tin and cover with cling film, and put in the fridge overnight.
A short time before you are ready to serve the cake, take it out of the fridge.
Spring the cake free, then transfer to a plate without removing the base.
Push the cocoa through a sieve to dust the top of the cake and dot over the fresh raspberries.