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