Heat a pan over medium heat and when the pan is hot add the cumin seeds and whole pepper and dry roast it for about a minute or until the aroma of cumin seeds come out.
Turn off the heat and add turmeric powder to the pan and mix it. Allow it to cool. (This step is optional. But heating turmeric powder increases the solubility of curcumin. You don't need to mix it and heat it again, just adding it to the hot pan is sufficient.)
In a blender, add the roasted spices along with turmeric powder and mint. Pulse a few times.
Then add the yogurt, water, and salt. Blend it smoothly.
I usually don't strain the buttermilk mixture. But you can strain it and serve it chilled.
Instead of cumin seeds and whole pepper, you can add 3/4 tsp of cumin powder and 1/2 tsp of pepper powder. But as always adjust according to your preference.
If you are using store-bought buttermilk, I would recommend diluting it with water. For two cups of buttermilk, you can add 1 cup of water and dilute it. The buttermilk drinks are usually thinner in consistency, but if you prefer thick consistency, you can skip diluting it. You can use 2.5 cups of buttermilk for the spice measure.
Adjust the water according to your consistency preference and also the spices and salt according to your taste preference.