Take corn flour in a mixing bowl, add 1/2 cup milk to it and whisk it well without any lumps.
Take mustard seeds and powder it coarsely, I used my hand mortar and pestle to do this, set aside.
Take remaining 1/2 cup milk in a pan and heat it up.
Once milk is boiled, add corn flour milk mixture and keep stirring with a whisk. Keep in low flame and keep cooking.
Keep cooking it until its thick and creamy. Make sure the flame is low.
Keep whisking until its thick. Cool down completely and then blend it to a smooth paste.
Now transfer this to a bowl, add pepper powder, mustard seeds powder, sugar, salt, olive oil and vinegar.
Whisk it well until its creamy.
Store Mayonnaise in airtight container and refrigerate it.