Pulse the cocoa powder and salt in the bowl of a food processor several times until smooth.
Whisk together the water, sugar and corn syrup in a medium saucepan and bring to a boil over medium heat.
Turn the food processor on and let run continuously while carefully pouring the boiling-hot sugar syrup down the feed tube. Follow with the vanilla and cardamom.
Cool 15 minutes before transferring via funnel into a large squeeze bottle. Then cool completely before refrigerating for up to 3 months. Or divide into two containers, refrigerating one, stashing the other in the freezer where it will keep indefinitely? Unless of course you come back and eat it the next day.
After grinding the cardamom finely in a spice grinder, you'll have to sift out the fibrous threads. Yes, it's worth it.