Heat the milk in a small pot on medium heat until warm. Remove from the heat, add the chiles and soak for about 20 minutes. Transfer to a blender and process until smooth.
Melt the butter in a medium saucepan over medium heat. Add the flour and cook for 1 minute. Whisk in the chile-seasoned milk (the color will be red) and cook slightly until thickened. Remove from the heat and stir in the cheese. Season with salt.
Serve immediately with chicharrones, tortilla chips or pretzels.