In a small saucepan, heat the maple syrup and let it simmer about 5 minutes until it is reduced to about 3/4 cup. Set aside to cool.
In a medium saucepan, heat the milk, half and half, and salt over medium/low heat until it just comes to a boil, about 4 minutes.
While the milk/cream is heating, whisk the egg yolks ina medium bowl for 2-3 minutes until lighter in color.
When the milk is simmering, remove it from the heat and pour 3/4 cup into the egg yolks, whisking constantly as you pour. Then pour the egg yolk mixture back into the saucepan with the rest of the milk mixture. Put the saucepan back over medium heat and stir constantly for about 5 minutes, until the custard thickens and coats the back of a spoon. (When you run your finger through the custard coating the back of the spoon, you should be able to make a line that doesn't run back on itself.)
Remove the custard from the hea, and stir in the maple syrup.
Pour the custard through a fine-mesh strainer into a mixing bowl. Carefully place the bowl into a larger pan half-filled with ice and water. Let the custard chill for about 45 minutes.
Cover teh custard and refrigerate overnight or at least 4 hours.
When the custard is well-chilled, freeze it in an ice cream maker according to the manufacturer's instructions.
Optional: add toasted walnuts when the ice cream is almost fully churned and place into the freezer until the ice cream is very firm.