Set vanilla ice cream out on the counter to soften. Remove the lid so it will soften quicker.
Whisk together pistachio pudding mix with 2 c. cold milk in a bowl. Place it in the fridge to set up while you're preparing everything else.
Preheat oven to 350 degrees Fahrenheit. For the crust, use a fork to combine flour, chopped pistachios and softened butter in a bowl until no loose flour remains and coarse crumbs form. Press into 9"x13" pan. Bake for approximately 8-10 minutes. Allow it to cool while you're preparing the ice cream layer.
Stir together prepared pistachio pudding (that's been setting up in the fridge) with softened vanilla ice cream until it's all incorporated. Spread over cooled crust. Cover with foil and place in the freezer for approximately 4-6 hours to allow the ice cream to re-freeze.
Just before serving, whip the heavy whipping cream with powdered sugar and vanilla in the bowl of an electric mixer with the whisk attachment on, until stiff peaks form. Spread over ice cream layer, garnish with 1/4 c. chopped pistachios and serve immediately.