Line the baking sheets with parchment and draw and 8-inch circle in each one with a bow or cake pan and use that to outline it.
Whisk egg whites until stiff, then add sugar a spoonful at a time, beating in well after each addition. It takes patience. Sprinkle over the cocoa and vinegar and fold in gently but firmly.
Divide the meringue disks among the 3 circles, spreading evenly. Don’t worry about beating the air out of them.
Cook for 1 hour, then turn off the oven, leaving the meringues until cool. If you keep them in an airtight container in-between parchment paper they will keep for a week.
Now for the creme patissiere: Beat the egg yolks and sugar togethe, then add the cocoa and flour, whisking well. Warm the milk and cream in a saucepan, then, whisking, pour this onto the eggs and sugar before pouring everything back into the saucepan on the heat, stirring constantly, bring it to a boil. When the mixture has thickened, take it off the heat and stir in the melted chocolate and vanilla.
Let it cool now, but avoid putting it in the refrigerator as it will become too solid. You can stop it forming a askin either by covering it with buttered parchment paper. Another option is to stick it in a bowl of ice water and keep stirring it until it is cold.
To assemble the cake place one of the meringues discs on a flat plate or cake stand, spread with a third of the chocolate creme, then carry on layering. Spread the top with chopped nuts of your choice.