It isn't my Grandma's recipe but it isn't a secret either since it was posted by a member on BYH:
https://www.backyardherds.com/threads/devonviolet-acres.30335/page-260#post-524015
Note the reduction of booze. Great Grandma Wall must have had a lot of people staggering around on Christmas Day! We've not determined what size shot "Grandma" used. I put in 6 of the larger side from a 2 'sided' jigger ~1.3 oz each. Potent enough for us! I replaced the "missing" liquid with DW's nonfat Lactaid milk.
It is really smoooooth, especially after a couple of days in the refrigerator though it does lose some of its "loft". It also separates (no surprise) so a good shake is necessary.
Here are the pictures I posted when I made it:
https://www.backyardherds.com/threads/devonviolet-acres.30335/page-266#post-526196
I enjoyed reading those posts, lol.
The one I make is liquor-free (but it can be doctored to anyone's liking). Here's my recipe:eggnog-anytime.68099
Quite a bit different, your's sounds like less hassle (unless you decide you want to beat the ingredients by hand again).