In a medium bowl, combine the graham cracker crumbs and melted unsalted butter until fully mixed. Press the mixture into a 9x13-inch pan with a flat-bottomed measuring cup to create an even crust. Chill in the refrigerator for 15 minutes to firm up the base while you work on the next layer.

In a mixing bowl, beat the softened cream cheese and powdered sugar together until smooth and creamy, for about 2-3 minutes. Gently fold in half of the thawed Cool Whip until combined, ensuring a light texture. Spread this mixture evenly over your chilled graham cracker crust.

Drain the crushed pineapple, retaining the juice. Evenly spread the crushed pineapple over the cream cheese layer, allowing the juices to seep slightly into the cream.

In a medium bowl, whisk together the instant pistachio pudding mix and cold milk. Stir vigorously for about 2 minutes until thickened and creamy. Gently fold in the remaining Cool Whip, then spread this pistachio layer over the pineapple.

Cut the baked and cooled yellow cake into squares or cubes. Gently place the pieces atop the pistachio layer, covering it evenly.

Cover the pan tightly with plastic wrap or aluminum foil and refrigerate for at least 4 hours, or ideally overnight.

When ready to serve, remove the cake from the refrigerator. Top with additional Cool Whip, whipped cream, or a sprinkle of chopped pistachios. Slice into squares and enjoy.