1.5 cups whole wheat flour
1.5 tsps baking powder
1/2 tsp baking soda
3 tbsps cocoa powder
1 tsp custard powder
1/2 cup potato puree [cold]
½ cup + 1tbsp milk
2 tbsps honey
1/2 cup+ 2 tbsps powdered sugar
1/3 cup cooking oil
1/2 tsp vanilla essence

As a substitute for egg mix 1.5 tsps of cornflour + a pinch of baking powder + a pinch of baking soda + 2 tbsps of water

Preparing the potato puree- Pressure cook 1 large potato for 4-5 whistles. Allow to cool completely before peeling and roughly mashing the potato. Give it a light churn in an electric mixer. This will get rid of any lumps and give you a sticky potato puree. Store this in the refrigerator. [Use 1/2 cup of the prepared puree once it is chilled]
Pre heat the oven at 180 degrees for 10 minutes.
Sieve the dry ingredients [whole wheat flour, baking powder, baking soda, cocoa powder and custard powder] together 5-6 times.
In a large bowl cream the oil and sugar together. Then add the egg substitute, honey and vanilla essence and mix well.
Stir in the potato puree.
Add the milk and beat the mixture for about 5 minutes. This will make sure that all the wet ingredients are combined well.
Fold in the sieved flour gently until well combined. [The batter is slightly thick similar to a brownie batter . You may add a spoon or two of milk if you think it is too thick].
Carefully spoon it into a greased and lined baking dish. [I used an 8 inch round tin]
Smoothen the top with a spatula.
Bake in the pre heated oven at 180 degrees for 30 minutes or until the skewer inserted comes out clean.
Transfer onto a wire rack after 10 minutes. 
Slice once completely cooled