Wash and grate the cucumber. Transfer into a sieve over a bowl to drain.
In a bowl add the yoghurt and give it a mix. Then add in the grated cucumber.
Season with Kala namak, You can adjust the salt to your taste. Add roasted cumin powder and give it a good mix.
In a small pot or pan, heat the oil. Bloom the cumin seeds and then add chilli powder. As soon as you add the chilli powder, remove from heat and pour the tadka on top of the cucumber-yoghurt mix.