Grease the back of a stainless steel plate/ flat pan with ghee or line a baking sheet with parchment paper.
Heat a heavy bottomed pan over medium heat. Add 1 tsp ghee, the grated jaggery and 1 tbsp water. Give this a good stir.
Cook this mixture, stirring continuously for about 8 to 10 minutes, until the jaggery mixture starts to melt, soften and then caramelize. Make sure the heat is not high so that the mixture doesn't burn.
You must look out for the hard ball stage which is the point where a drop of the jaggery mixture when dropped in cold water sinks and hardens into a toffee.
Switch off and remove the pan from heat.
Immediately fold in the roasted peanuts, salt and cardamom powder if using, and stir until combined well.
Quickly thereafter, transfer this mixture onto the back of the plate or on the parchment paper, and using a rolling pin, roll this to an approximately ¼ inch thickness.
Let this now set until cool and you can then break into rough shapes using your hand after about an hour.
You may also cut across the set mixture with a knife or pizza cutter while still warm to allow symmetrical shapes to come out later.
Store the gajak or brittle in an airtight container.