Cover pistachios in cold filtered water and refrigerate overnight. The next day, drain and rinse the pistachios. Add them to a blender or food processor with 2 cups filtered water and 2 cardamom pods. Puree on high speed for 1 minute.
Drain pistachio milk through a nut milk bag or several layers of cheesecloth. Use within 3 days. Yields 2 cups.
To make hot chocolate:
Heat pistachio cardamom milk over medium heat. Ladle a small amount of the milk into a bowl with the cocoa powder and whisk until smooth, then pour the mixture into the pistachio milk. This will prevent the cocoa from lumping.
While the mixture is heating, whisk in agave nectar, ground cardamom, vanilla and salt. Once the mixture is hot, serve immediately.