Soak 200 grams of pistachios in water for 4-5 hours. After soaking, peel the skin and grind the pistachios into a smooth paste using a little water.
In a heavy-bottomed pan, heat 100 grams of ghee on medium flame. Add the pistachio paste and cook for 5-7 minutes, stirring continuously to avoid sticking to the pan.
In another pan, prepare sugar syrup by boiling 200 grams of sugar with 1/2 cup of water. Cook until the sugar dissolves completely and the syrup reaches a one-string consistency.
Slowly pour the sugar syrup into the cooked pistachio paste, stirring continuously to mix well. Cook for another 5-7 minutes until the halwa thickens and starts leaving the sides of the pan.
Add 1/2 teaspoon of cardamom powder and mix well. Cook for another 2 minutes.
Grease a plate or tray with ghee and transfer the pista halwa onto it. Spread it evenly and let it cool for a few minutes.
Garnish with chopped pistachios and press them gently into the halwa. Allow the halwa to set for 1-2 hours before cutting it into desired shapes.