In a blender or food processor, process the leeks and onion until finely chopped.
In a large saucepan, heat the olive oil over medium heat. Add the chopped leeks and onion, and saute until tender and opaque (do not brown), about 5 to 7 minutes. Add the potatoes, chicken broth and mace. Cook over medium heat until the potatoes are tender.
Pour the mixture into the blender or food processor. Blend until smooth. Pour into a large bowl and refrigerate.
Before serving, stir in the evaporated milk. Ladle into individual bowls. Top each with 1 tablespoon chives and pepper to taste. Serve cold.