Mix milk(at room temperature) with vinegar and keep it aside for few minutes to curdle.
Add oil to the above mix , keep it aside
Sift flour and baking soda and sugar for 2-3 times
Add the dry cake mix to the liquid and mix well
Keep 1 cup of cake mix aside, add cocoa powder to the remaining mix.
Add 2-4 tbsp. water as required to make the cocoa cake mix of right consistency
Grease the cake tin(preferable wide and square/rectangle) with oil/butter.
Pour into the prepared cake pan.
Bake for 30-35 minutes(at 160 degree C) or till a knife tip comes out clean.
Let it cool completely and turn the pan upside down on a large plate and tap slightly on the bottom of the pan to unmould the cake.
Keep it aside on a wire rack to cool.