In a large bowl, mix cornflour with water and whisk well to avoid forming lumps.Add food color and keep aside.
In a non stick pan, dry roast the chopped cashews and keep aside to cool.
In a non stick wok, heat 1/2 cup of water and add sugar and boil till the sugar melts. Cook for another few minutes till the water strats getting a little thick. No need to wait for one string consistency.
Now start adding the cornflour mixture by stirring continuously in low heat. You will see the mixture turning dark in color and the texture will be initially look glossy.
Keep stirring till the mixture starts getting thick. Now add 2 tbsp of ghee and roasted cashews.
Stir it till the mixture folds up and starts leaving the sides of the wok forming a big lump.
Transfer the mix into a greased tray and tap gently so that the mix spreads evenly. Keep aside to cool down completely.
Once the halwa cools down completely in room temperature cut into desired pieces. I used pizza cutter to get fine edges.
Serve it by garnishing chopped cashews and pistachios.
The karachi halwa can be consumed till 10 days if stored in refrigerator.