Line a large sieve with cheesecloth; set over a medium deep bowl. Place 1kg of yogurt & half teaspoon of salt into cheesecloth on sieve. Gather edges of cheesecloth to cover yogurt. Place in refrigerator and let drain. Discard excessive liquid from bowl time to time.
Next day remove and add half teaspoon salt into yogurt mixture again. Wash cheesecloth and repeat steps for 3 days.
Finally, gently squeeze out any excess liquid; discard liquid in bowl. Yogurt will have a thick consistencies. Whisk in cracked black pepper, chili flakes, lemon zest and a pinch of salt. Transfer mixture into an air tight jar filled with olive oil and a piece of bay leaf.
Cover; place in refrigerator and let marinate for at least 8 hours and up to 2 weeks.