Add 1 tbsp of walnut oil to the chickpea flour and mix it in with your fingers until the besan assumes a slightly grainy texture. Set this aside for about 10 minutes.
Heat the remaining walnut oil in a saucepan with sloping sides or a kadhai. Add the besan and cook over medium-low heat, stirring frequently, until the besan gets a couple of shades darker, around 8 minutes.
Add the powdered cashews and again stir-fry for another 5 minutes or until the besan is cooked and a deeper yellow. You want to stir frequently so that the besan doesn't burn.
Remove the besan to a plate or a bowl and set aside while you prepare the sugar syrup. To make the syrup, mix the water and the sugar and the powdered cardamom in a saucepan and cook until the sugar has dissolved and the mixture bubbles.
Remove from the heat and mix in the besan. Stir fast so the besan doesn't lump up. When you have a smooth mixture, remove the halwa to a bowl.
Garnish with cashew nuts. The halwa will firm up a little as it cools. Scoop it with an ice-cream scoop or a spoon to serve.