Light the ptex candle slowly so that when lit it glows blue NOT orange. Orange flame means there's carbon in it which means it will brown on the base.
Once you have it glowing blue, hold it over the hole in your base very close. You want the ptex to flow in, not drip. When you drip clear, it picks up contamination and browns. So hold it close enough that the blue flame just barely touches the base and the base will kind of pull the molten ptex off the candle.