Thaw Cool Whip in the refrigerator for 4 hours or overnight.
Melt butter and mix with crushed graham crackers and sugar. Press into the bottom and up the sides of a 9" pie tin. To set the crust, bake in a 350*F oven for 3 ½ minutes (optional).
In a bowl whisk together pudding mixture and COLD milk. Whisk for 2 minutes.
Using a spatula, combine the Cool Whip with the pudding mixture using a folding motion, until well combined. Pistachio pudding has little bits of pistachio in it, so there will be tiny chunks of pistachio. See photos above for a visual.
Pour into graham cracker crust and smooth out. Chill for 2-24 hours. Can be served immediately, but the filling will be extremely soft and pudding-like.
Decorate with whipped cream and pistachio bits and/ or toffee bits if desired.
Video
Notes
This recipe is for a typical 9" pie tin. If you have a deeper dish pie plate, double the recipe.
Store in the refrigerator for up to 2 days. It won't go bad for a few more days after that, but the pudding will start to weep into the crust making it mushy and less delicious.
Freeze! This can be made ahead and frozen. Freeze right after making. Freeze for up to 1 month. Whole milk is recommended for this, as the less fat in milk, the more likely there will be little ice shards in the pie.
Thaw for at least 1 hour at room temperature before serving.
Make sure your Cool Whip is completely thawed before folding it into the pudding for best results.
Make sure to use a folding motion and to not stir in the Cool Whip. Stirring it will deflate it, folding will ensure it stays fluffy.