01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan with butter or nonstick spray.
02 - Prepare brownie batter according to package instructions. Pour into prepared pan and bake for 25-30 minutes until a toothpick inserted in the center comes out with a few moist crumbs. Cool completely, then cut into 1-inch cubes.
03 - Whisk together pistachio pudding mixes and cold milk in a large bowl for 2 minutes until thickened. Add green food coloring if desired. Refrigerate until ready to assemble.
04 - In a chilled bowl, beat heavy cream, powdered sugar, and vanilla extract with an electric mixer until stiff peaks form.
05 - In a large trifle dish or glass bowl, layer half the brownie cubes, followed by half the pistachio pudding, then half the whipped cream. Repeat with remaining ingredients.
06 - Top with green sprinkles, chocolate shavings, or chopped pistachios if desired. Refrigerate at least 2 hours before serving for optimal flavor and texture.