Melt the butter in a small saucepan over medium heat. Whisk in the flour and cook for 1 minute. Add the milk, increase heat to high and cook, whisking constantly, until slightly thickened, about 5 minutes. Remove from the heat and whisk in the Chihuahua cheese until melted. Stir in the Parmesan and season with salt and pepper. Thin with additional milk if needed. Keep warm.