Preheat oven to 180 degrees C. Grease and flour a 23cm springform tin.
In a large bowl, mix flour, 1 cup sugar, cocoa and 1/4 teaspoon salt. Make a well in the centre and add the coffee, oil, 1 tablespoon vanilla and egg. Beat until combined. Batter will be thin.
Pour into springform tin. Bake 30 to 45 minutes or until skewer inserted into cake comes out clean. When cake is completely cool, remove from pan, cut top of cake level with a knife, and place back into springform tin.
To make the mousse: melt 450g chocolate and 125g butter over double boiler, stir in 1/4 teaspoon salt and 2 teaspoons vanilla, whisk in yolks. In a separate bowl, beat whites until foamy, gradually add sugar and continue beating until stiff. Fold into chocolate mixture. Whip the 1 1/2 cups cream in a separate bowl and fold into chocolate mixture. Pour over leveled cake in springform pan and let set for 1 to 2 days in the fridge.