Coarsely grind the walnuts in a chopper or food processor. don't make fine powder.
In a non stick pan take 3 tbsp ghee and add the crushed walnuts and roast on low heat for 2 minutes.
Now add milk and cook till it thickens slightly.
Now add the grated khoya and cook till the khoya melts and got mixed with the walnuts mixture.
Now add sugar and coco powder and cook on medium heat.
Keep stirring continuously else it may stick and burn at the bottom.
When the mixture thickens and start leaving the sides of the pan then add cardamom powder.
Grease a metal or glass plate and pour the prepared barfi in the plate and smooth out the surface it with a spatula.
Sprinkle crushed pista And almond on top and press gently.
Let it cool down for 1/2 hour and then cut into squares.
When It cools down completely then store in a box.
Stays well for a week at room temperature.