Preheat oven to 180 degrees C. Grease and flour a 23x33cm cake tin.
Melt the chocolate in a double boiler or in the microwave.
Pour the melted chocolate into a large mixing bowl. Blend in the mayonnaise, vinegar and vanilla. In a separate bowl combine the flour, sugar, bicarbonate of soda and salt then gradually stir into the chocolate mixture. Slowly stir in the warm water. Pour mixture into prepared tin.
Bake in the preheated oven for 35 to 40 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
The thought of Mayo in a cake was initially ew, but then remembered Mayo is make of oil and eggs so gave it a go and really glad I did - best cake ever. I used normal cooking milk chocolate and it didn't seem chocolate-ie enough so I put in the the same amount again of dark chocolate. This will be the only chocolate cake I ever make. - 22 Oct 2013