Blend shelled pistachios, milk powder, powdered sugar, and milk to a smooth paste.
Roast kataifi/vermicelli in a pan with 2 tbsp ghee over medium flame until golden brown. Transfer to a mixing bowl, add 3-4 tbsp pistachio paste, hazelnut paste, and chopped pistachios. Mix well and set aside.
In the same pan, cook remaining pistachio paste with the extra milk powder over medium heat, stirring continuously until slightly thickened. Add crushed cardamom and 2 tbsp ghee, continue stirring until the mixture forms a non-sticky dough and leaves the pan. Remove to a plate and let it cool.
Grease the insides of your modak mold with ghee, sprinkle in rose petals and slivered pistachios, and press a layer of pistachio dough inside, creating a hollow center.
Stuff with the kataifi-pistachio-hazelnut filling, seal with more pistachio dough, then demold and set aside. Repeat with remaining mixtures.
Melt chocolate chips in the microwave in short bursts, stirring in between until smooth and glossy, or use a double boiler for gentle, even melting. Add ghee if you are using baking chocolate for glossy texture.
Dip the tip of each modak halfway into the melted chocolate, garnish with slivered pistachios and rose petals, and set aside to cool until the chocolate sets.