01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan with butter or nonstick spray. Prepare brownie batter according to package instructions, pour into prepared pan, and bake for 25-30 minutes until a toothpick inserted in center comes out with moist crumbs. Cool completely, then cut into 1-inch cubes.
02 - In a large bowl, whisk together pistachio pudding mixes and cold milk for 2 minutes until thickened. Add green food coloring drop by drop until desired vibrancy is achieved. Refrigerate until ready to assemble.
03 - Chill a mixing bowl and beaters. Beat heavy whipping cream, powdered sugar, and vanilla extract with electric mixer on high speed until stiff peaks form and cream holds its shape.
04 - In a trifle dish or large glass bowl, layer half the brownie cubes evenly across bottom. Spread half the pistachio pudding over brownies, then top with half the whipped cream. Repeat layers with remaining ingredients, ending with whipped cream on top.
05 - Sprinkle green sprinkles, chocolate shavings, or chopped pistachios over top layer. Refrigerate for at least 2 hours before serving to allow flavors to meld and layers to set.