This is from my Pie book.
1 large, ripe avocado, halved and pitted
1 14-oz can sweetened condensed milk
2 tablespoons fresh lemon juice
2 tablespoons fresh lime juice
4 3-oz packages cream cheese, softened
1/3 c. powdered sugar
Spoon the avocado flesh into a blender. Add the condensed milk, lemon juice, and lime juice and process until smooth. Set aside.
In a large bowl, beat the cream cheese with an electric mixer until smooth and fluffy. Beat in the powdered sugar, then blend in the avocado mixture, adding it about one-third at a time. When the mixture is smooth, pour into a graham cracker pie shell. Cover with loosely tented aluminum foil and refrigerate until firm, about 6 hours. Garnish with whipped cream.