In a medium bowl, add the milk and bittersweet chocolates. Pour the hot coffee over the chocolate, cover the bowl with plastic wrap and set aside for 5 minutes.
Pour the cream and milk into a microwave-safe bowl or liquid measuring cup and microwave on high heat until warm, 30 seconds to 1 minute. Pour the warm-milk mixture over the chocolate, add the corn syrup, vanilla and salt and stir to combine. Fill each of the sections of a six-pop ice mold with the chocolate mixture.
Freeze the mocha pops until they're semi-frozen, about 2 hours. Insert a popsicle stick into the center of each one, return to the freezer and freeze until hard, another 6 hours or overnight. To remove the mocha pops, place the bottom of the mold under hot running water until a pop easily slides out (if the mocha pop looks a little melted around the edges, then place it on a parchment-paper-lined baking sheet and refreeze until hard). Serve.