Heat the oil in a small fry pan; add the cumin and black mustard seeds, stir and cook over medium heat for a few minutes. remove from heat and add the sugar and chaat masala. Mix well.
Add the spiced oil mixtue and mayonnaise (or yogurt); salt and pepper. Mix well.
Shred the cabbage and carrots and place in a mixing bowl. Add the spiced mayonnaise or yogurt dressing and mix well.
Cover and set aside in the frig until ready to serve.