Place the coffee in a medium saucepan. Bring to a boil, reduce heat and simmer until it reduces to about ½ cup. (~10 minutes) Set reduced coffee aside.

Place the milk in a medium saucepan. Heat over medium-low until it just begins to simmer.

While the milk is heating, whisk together the salt, sugar and egg yolks in a medium bowl. Gradually pour the hot milk into the egg yolks; whisking vigorously the entire time.

Return the mixture to the saucepan and cook over medium-low heat, stirring constantly, until the mixture thickens noticeably and coats a wooden spoon. (~4-5 minutes) (Note: If you have a kitchen thermometer, the target temperature is 180°F. Do not allow mixture to boil.)

Remove mixture from heat and add the heavy cream, vanilla extract and reduced coffee; stir until well combined. Pour the mixture through a fine-mesh strainer into a clean container. Cover with plastic wrap, pressing the plastic directly onto the surface of the mixture, and refrigerate overnight.

Pour the ice cream mixture into an ice cream maker and freeze according to the manufacturer's directions.

(Optional) If you desire a firmer ice cream, transfer the finished ice cream into the freezer for a couple of hours.