Take a bowl and add oil in it.
Add sugar and whisk together very well. (See notes below)
Put a sieve on the bowl and add whole wheat flour, custard powder, salt and baking powder.
Sift the Ingredients into the bowl.
Mix the ingredients using a spatula.
Now begin to add milk but little at a time. Keep mixing.
A thick batter of flowing consistency is ready.
Lastly add lemon juice and give a last mix.
Transfer the batter into a greased cake tin lined with butter paper. The size of tin is 9 inches.
Bake @180°C for 30-35 minutes.
Once done, bring it out and check by inserting a toothpick. If the toothpick comes out clean, then it means cake is baked properly.
Let it cool down completely.
Invert and enjoy after cutting into pieces of desirable size.