We've been unable to track down an exact history of this unusual dessert. The beginnings of the recipe (at least in the U.S.) seems to be California. This is no surprise given California has historically been the largest grower of avocados in the U.S. The earliest recipe we've found so far dates back to 1932 from the Calvo Growers of California published in the "New Calavo Hostess Book". This version includes milk, cream, egg white, fresh avocado, vanilla, sugar and vanilla.
We found yet another early version in the San Francisco Examiner Cookbook from 1947 which strengthens our California lineage theory. That being said you can also find avocado ice cream in other countries like India or the Philippines.
Since those early basic recipes, many ingredient variations have surfaced including some that use pre-made ice cream as a base and others with additions of chiles, tequila, orange juice or coconut.
In our humble opinion the best recipes are the ones with the most basic of ingredients given the avocado has such a delicate flavor it can be easily overwhelmed.
Adapted from Avocado.org
Beat yolks and sugar until thick; reserve.
Heat milk to boiling, stirring frequently.
Temper reserved egg mixture with a little hot milk; stir tempered mixture into remaining hot milk.
Heat mixture over low heat to 185 degrees F, stirring constantly.
Remove from heat; continue to stir a few minutes; stir in cream. Thoroughly chill mixture.
Meanwhile, pit and peel avocados. Puree in a food processor with lime and lemon juices.
Whisk avocado puree into chilled custard. Freeze in an ice cream freezer following manufacturer's instructions.
Store in an airtight container in the freezer.