Preheat your oven to 350°F (175°C) and lightly grease a 9x13 inch baking pan.
In a large mixing bowl, combine the white cake mix, the first instant pistachio pudding mix, the first Dream Whip mix, eggs, water, vegetable oil, and almond extract. Beat on low speed for 30 seconds, then scrape down the bowl. Increase to medium-high speed and beat for 2 minutes until smooth and well combined.
Pour your lovely green batter into the prepared pan. Bake for 25-28 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely on a wire rack – patience is key for perfect frosting!
In a separate mixing bowl, combine the cold milk, the second instant pistachio pudding mix, and the second Dream Whip mix. Start mixing on low speed until everything is just combined, then scrape down the bowl. Increase to high speed and beat for 4-6 minutes until the frosting is wonderfully fluffy and forms soft peaks.
Once the cake is completely cool, gently spread the airy pistachio frosting evenly over the top. Serve your delightful Pistachio Sheet Cake right away, or pop it in the fridge until you're ready to enjoy!