Preheat oven to 350F. Grease a 9x13" baking pan with nonstick spray and set aside.
In the bowl of a stand mixer fitted with the paddle attachment (or in a large mixing bowl if using a handheld electric mixer), combine cake mix, dry pistachio pudding mix, vegetable oil, eggs, almond extract, and undrained crushed pineapple.
Mix on low speed for about 30 seconds, then scrape down the sides of the bowl. Increase speed to medium and mix for another couple of minutes.
Pour the batter into the prepared pan, spreading it evenly.
Bake in preheated oven for about 35-37 minutes, or until a toothpick inserted in the center of the cake comes out clean.
Remove cake from oven and let cool completely.
To prepare the frosting, combine milk and pistachio pudding mix in a medium mixing bowl. Whisk for 2-3 minutes, or until pudding mix has dissolved and the mixture is beginning to thicken. Let it sit for a few minutes.
Gently fold the whipped topping into the pudding mixture, until it's fully combined and no streaks remain.
Spread frosting evenly over the surface of the cake, then refrigerate for about two hours before serving to allow the frosting to set up.
If desired, sprinkle cake with chopped pistachios, then slice and serve!