01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with butter or nonstick spray, ensuring all corners and sides are coated.
02 - Prepare brownie batter according to package instructions, adding required eggs, oil, and water. Spread evenly into prepared pan. Bake for 25 to 30 minutes until a toothpick inserted in the center emerges with moist crumbs. Cool completely in pan, then cut into 1-inch cubes.
03 - In a large mixing bowl, whisk both packages of instant pistachio pudding mix with cold milk for approximately 2 minutes until mixture thickens. Stir in 1 to 2 drops green food coloring if a more vibrant hue is desired. Refrigerate until assembly.
04 - Chill a metal bowl and beaters in freezer for 5 minutes. Combine heavy cream, powdered sugar, and vanilla extract. Beat with electric mixer on high speed until stiff peaks form, taking care not to overwhip.
05 - Begin assembly in a 3-quart trifle dish or clear glass bowl. Spread half the brownie cubes across bottom. Top with half the pistachio pudding, smoothing evenly. Add half the whipped cream in a smooth layer. Repeat sequence with remaining ingredients, finishing with whipped cream on top.
06 - Sprinkle green sprinkles, chocolate shavings, or chopped pistachios over the whipped cream layer. Refrigerate for at least 2 hours or up to 24 hours to allow flavors to meld and texture to set.