Sieve Maida, cocoa & baking powder atleast thrice.
Mix baking soda in buttermilk & keep aside.
Dissolve the powdered sugar in the oil.
Now mix the sieved mixture & buttermilk alternatively to the oil & sugar mixture. Beat well.
Add vanilla essence & mix again.
Transfer this mixture into a pre-greased cake pan & bake it in pre-heated oven at 350oC for about 35 to 40 minutes.