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