Peel the cucumbers, trim the ends, cut in half lengthwise and scrape out the watery area where seeds would be. Coarsely chop. Crush the garlic and 1/4 teaspoon of the salt into a paste with the flat side of a chef's knife. Combine the cucumber, garlic, remaining salt, and the buttermilk in a blender and process until completely smooth. Add half of each of the herbs and the cayenne and pulse several times to combine. Transfer soup to a bowl, cover, and refrigerate for 1 hour or until thoroughly chilled. Before serving, season with salt, pepper, and lemon if desired. Divide among 4 soup bowls and sprinkle with the remaining herbs. Serve.