Zest your limes and divide zest in half, set aside.
Juice the limes into a measuring cup and using fresh or bottled key lime juice brig to 3/4 cup and set aside.
In the bowl of an electric mixer, beat the softened cream cheese, sour cream, ricotta cheese, lime juice, xylitol, vanilla extract, 1/4 cup hot water, and zest of 1 lime on medium speed until well combined.
Place 3/4 cup boiling water into a small bowl or cup. Using a small whisk or fork to mix, slowly pour gelatin to the hot liguid thoroughly dissolving the 3 envelopes of gelatin. Do not let cool and move onto the next step immediately. (It is important that the water be boiling hot and that you mix the gelatin powder in quickly so it is completely dissolved. If not, you will have gummy bear-type lumps.) With the mixer on high and working very quickly so the gelatin does not set, blend the hot dissolved gelatin thoroughly into cheesecake mixture. Immediately pour mixture into the prepared pie crust and sprinkle with the zest of the other lime. Chill in the refrigerator for about 3 to 4 hours or until firm. Serve.
Cook’s Note:
If you have any extra cheesecake mixture left over after filling the pie pan you put it in a pastry bag or heavy-duty plastic bag and put it in your freezer for about 7 to 10 minutes, just to firm it up a bit to the thickness of whipped cream. Then take it out and decorate the top of the Key Lime Cheesecake. (If it is in a plastic bag, snip the corner off and squeeze it on.)