Pulse the cocoa powder and salt in the bowl of a food processor several times until smooth.
Whisk together the sugar, water, and corn syrup in a medium saucepan and bring to a boil over medium heat.
Turn the food processor on and let it run continuously while carefully pouring the boiling-hot sugar mixture into the feed tube. Follow with the vanilla and cardamom.
Let cool for 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.