Pick out any bitter shell fragments. Pulse walnuts in a food processor to a coarse meal (like fine gravel), not a sticky paste. A few larger bits are fine.

Heat ghee in a wide heavy pan over medium heat until shimmering. Add ground walnuts and cook 1 to 2 minutes, stirring constantly, until color deepens slightly and the kitchen smells toasty. Lower heat if the mixture smokes.

Pour in milk; stir until no dry pockets remain. Whisk in milk powder if using. Cook on medium-low 6 to 10 minutes, stirring often, until milk is roughly half reduced and the mixture looks thick and glossy, like wet sand that holds a line when you drag a spoon through.

Add sugar; stir until dissolved. Cook on low to medium-low 8 to 12 minutes, scraping the sides, until the halwa pulls together and thin ghee streaks appear at the edges.

Remove from heat briefly if the pan is very hot, or reduce to low. Fold in chopped chocolate until melted and streaky-uniform. Taste; add a tiny pinch of salt if the chocolate was very sweet.

Spoon into small bowls. Top with walnut pieces. Serve warm or chilled.