Pre-heat oven to 180 degrees. In a blender, mix the milk, oil, eggs and sugar for 2 minutes. Transfer to a bowl and mix in the sifted flour, then the sifted cocoa and finally the baking soda and baking powder. Pour in a greased and cocoa dusted pan and bake for 40 minutes. Remove cake from oven and let cool.
Combine the sour cream and melted dark chocolate. Beat well with a mixer and chill.
Cut the cake in half and spread the chocolate mousse in between the two layers. Refrigerate for 40 minutes then cover with dark chocolate ganache.