In a small saucepan, whisk together the cocoa powder, sugar, and salt. Pour in the water, stir to mix, and bring to a boil over medium heat, stirring constantly and scraping down the sides of the pan with a rubber spatula for an even texture. reduce the heat to a simmer and continue stirring and scraping for about 30 more seconds. Stir in the cardamom, remove from the heat, and let cool for about 10 minutes.
Use immediately, or let cool to room temperature and store in an airtight container in the refrigerator for up to 2 weeks.