This recipe works best with good quality cream, preferably from grass-fed cows with a fat content of 36% to 40%. Avoid anything that says "ultra-pasteurized" since it has been heat-treated, and you won't get the same results.
Most modern ovens go down as low as 175 to 180 degrees F (80 degrees C), ideal for this technique. A temperature of 200 degrees F (95 degrees C) will work, but check after 10 hours and see how things look.