Preheat oven to 350°. Pulse pistachios in a food processor until coarsely ground; reserve 1 tbsp. Add cookies, granulated sugar, and a pinch of salt to remaining nuts and whirl until finely ground. Add butter and whirl until mixture is just combined. Press over bottom and up sides of a 9- to 10-in. tart pan with a removable rim. Bake until fragrant, 10 to 12 minutes. Let cool completely on a rack.
Scoop avocados into cleaned food processor. Add cream cheese, powdered sugar, lemon juice, vanilla, and a pinch of salt; whirl until smooth. Spread filling in tart crust, preferably using an offset spatula. Chill while making ganache.
Put chocolate in a small heatproof bowl. Heat cream in a small saucepan until bubbles form at edge of pan. Pour cream over chocolate and let stand 1 minute. Whisk until smooth. Cool ganache until just warm to touch (95°), 5 to 6 minutes.
Pour ganache over avocado filling and gently spread to cover, preferably using a clean offset spatula. Sprinkle with reserved 1 tbsp. pistachios. Chill at least 2 hours.
MAKE AHEAD: Up to 8 hours, chilled and uncut (wrap airtight after 2 hours).