Toast anchos in a large skillet over medium heat, turning once, until they are softened, 4 to 5 minutes. Transfer to a saucepan; add water; bring to a boil. Reduce heat and simmer anchos 5 minutes. Remove from heat, let stand 10 minutes.
Remove anchos, reserving water. Discard seeds and stems. Pureé anchos and reserved water in blender until smooth. Transfer ancho mixture and remaining ingredients, except lemon and butter, to saucepan. Squeeze juice from lemon half into mixture; add lemon half. Bring sauce to a boil; reduce heat and gently boil 30 minutes.
Remove lemon half; stir in butter.Makes 4 cups Source: