In a small, heavy-bottomed saucepan, heat the sugar over a medium-low heat, until it begins to liquify. Continue to cook, without stirring, until the sugar is an amber color, about 2 minutes.
Add the hot water, and stir well, then transfer the mixture to six 4-ounce jars.
Place a second small, heavy-bottomed saucepan over a medium heat, and add the milk until just before scalding. Then, remove the milk from the heat, and transfer it to a bowl.
In a second bowl, whisk together the sweetened condensed milk, eggs and yolk, salt and vanilla. Add a little of the hot milk mixture to the egg mixture to temper it, and whisk constantly until well-combined. Repeat this once more before adding the egg mixture to the remaining milk mixture in a steady stream, whisking constantly to ensure the eggs don't scramble.
Divide the custard mixture between the jars, and seal the jars with lids.
Place the jars on a trivet inside a pressure cooker with 1 cup of water, lock the lid, and cook on high for six minutes.
Let the pressure release for 10 minutes before quick-releasing the rest.
Allow the jars to cool at least 30 minutes before transferring them to the refrigerator to cool for an hour before serving.
