In a medium saucepan, combine the whole milk, maple syrup, salt, and instant coffee.
Heat over medium heat until warm and steaming, but not boiling.
Remove from heat.
In a small bowl, whisk the egg yolks.
While whisking constantly, add one tablespoon of the warmed milk mixture at a time to the egg yolks.
When about half of the warmed milk has been whisked into the egg yolks, return the milk/egg yolk mixture to the pan with the remaining warmed milk.
Heat gently over medium-low heat until the mixture is thickened. Do not allow the mixture to boil! If you accidentally heat it too far and cook the eggs, strain the mixture through a fine-mesh strainer.
Place the finished coffee custard in the refrigerator until chilled.
When the custard is fully chilled, whisk in the heavy cream and vanilla extract.
Pour the mixture into an ice cream freezer and churn until frozen. If you don’t have an ice cream maker, you can pour the mixture into a freezer-safe container and stir it occasionally as it’s freezing.
Feel free to use decaf coffee if you're worried about the caffeine content.