Preheat the oven to 180 degrees C. Grease and flour a 23cm springform cake tin.
Sift together the flour, sugar, cocoa, bicarb soda and salt.
In a separate bowl, beat the eggs. Beat in the sour cream and vanilla until well mixed.
With an electric beater combine melted butter and oil. Add cold water and beat. Add the sifted ingredients all at once and beat at low speed 1 minute. Add egg mixture and beat another minute. Put batter in cake tin. Bake 50 minutes, or until it tests done.
Grind the chocolate in a food processor. In a saucepan, simmer the milk, sugar, instant coffee and salt until sugar dissolves. Pour into the food processor and whizz until chocolate is melted. Add vanilla and blend again.
Beat the cream to the soft peak stage. Fold a bit into the chocolate mixture to lighten it, then fold in the rest.
Cut the cake into two laters. Use half of the mousse to fill the cake, top with the other later, and use the other half to ice. Chill 3 hours before serving.