Step 1Using tongs, roast the chiles over a stove-top flame, turning them so the chiles blacken evenly on all sides, 3 to 5 minutes. (You can also place them under the broiler for a few minutes to blacken.) Put them inside a plastic bag about 5 minutes. Remove the charred skins, rinse and remove the seeds and veins. Set the chiles aside.
Step 2Peel the grapes and set them aside.
Step 3Melt the butter in a saucepan over medium heat. Add the shallot and cook until golden brown, about 5 minutes. Add the grapes to the skillet. Cook, stirring, until the grapes soften slightly, about 2 minutes. Remove them from the heat. Discard the shallot.
Step 4Blend the crema, yogurt and chiles in a blender until smooth, 1 to 2 minutes. Add the mixture to the saucepan and stir until it's well combined with the butter and grapes. Stir in 1/2 cup of milk if the soup is too thick. It should have the consistency of a creamed soup.
Step 5Chill the soup at least 2 hours. If the soup is still too thick, stir in a little more milk. Serve in a crystal bowl set in crushed ice.