Special equipment: blender
Add 4 cups of water to a medium pot over high heat and bring to a boil. Put the ancho chiles in a medium bowl and pour in the boiling water. Use a plate to weigh down the chiles so they remain submerged. Soak until softened, about 20 minutes. Add the chiles, garlic, 2 tablespoons of the chile soaking water, and the sour cream to a blender and blitz until smooth. Add more of the soaking water, if needed, for consistency. Season with salt and pepper, to taste. Transfer to a bowl or jar and store covered in the refrigerator.