Cut off stem and blossom ends of the zucchini and peel. (Remove all the green for a prettier color at the end). If the zucchini are very large, cut in half lengthwise and remove the seeds. If small, leave whole. (I cut the zucchini into chunks at this point).
Boil in a small quantity of water until tender OR you may microwave in a covered dish until tender, about 10 minutes. In either case, drain well, then mash or process to a smooth pulp.
Cook in the top of a double boiler over hot water with sugar, butter, lemon juice and zest. Stir frequently, until thick and creamy, about 15 minutes.
Ladle into hot sterilized jars, leaving 1/4 inch headspace. Wipe rims clean, put lids and rings in place and tighten down.
Invert jars for a few minutes, then return to upright and cool completely. Check seals, label and store in cool, dry place.
Note: It took 30 minutes for my batch to cook down. I processed my jars in a hot water bath for 5 minutes.