Roast and Blend the Pistachios:
In a medium-sized pan, dry roast the raw pistachios over medium heat for 3-4 minutes until fragrant. Be careful not to let them burn.
Once roasted, blend the pistachios in a mini food processor until finely chopped. Be sure not to over-blend to avoid creating pistachio butter. Set aside.
Prepare the Pistachio Cake (Using Boxed Mix):
Follow the instructions for the vanilla box cake mix (preferably one using butter).
In a large mixing bowl, combine the melted butter, milk, eggs, and cardamom powder. Mix well.
Add the dry cake mix ingredients to the wet mixture and stir until incorporated (but don’t over-mix).
Add green food coloring and ½ cup of the blended pistachios, folding them gently into the batter.
Butter a 9×13-inch baking dish and dust with flour to prevent sticking. Pour the cake batter into the dish and bake according to the package instructions (about 30 minutes).
Once baked, allow the cake to cool for about 30 minutes.
Prepare the Pistachio Milk Mixture:
In a large bowl, whisk together the evaporated milk, sweetened condensed milk, whole milk, cardamom powder, and ½ cup of the blended pistachios.
Add a few drops of green food coloring if desired. The milk mixture should be a light green color depending on the food coloring.
Reserve 1 cup of the milk mixture for serving later.
Once the cake is cooled, poke holes all over the cake using a fork, spacing them about 1 inch apart. Pour the remaining milk mixture over the cake, making sure it soaks evenly.
Cover the baking dish with plastic wrap and refrigerate for at least 6 hours, or overnight.
Make the Whipped Cream Topping (Next Day):
In a large mixing bowl, add the heavy whipping cream and vanilla extract. Use an electric mixer to blend at medium speed for 2 minutes.
Add powdered sugar and beat on medium-high speed until stiff peaks form. Be careful not to over-mix.
Assemble the Cake (Next Day):
Remove the cake from the refrigerator. Spread dollops of whipped cream over the cake, smoothing it evenly.
Garnish the cake with the remaining chopped pistachios and dried rose petals for a beautiful finish.
Serve cold, and for an extra touch, drizzle the reserved pistachio milk mixture over each slice before serving.