This Cucumber-Mint Buttermilk Soup can be made a day in advance and chilled in the refrigerator until ready to serve.
In a blender, puree buttermilk with cucumber and mint until smooth. Strain mixture through a fine sieve. Skim off any foam from surface. Season with salt and pepper. Refrigerate soup until cold, at least 2 hours and up to 1 day.
When ready to serve, stir until smooth. Divide among cups. Sprinkle with pepper, drizzle with oil, and top with mint leaves.