Bring milk and maple syrup to just below a simmer in a saucepan. In a separate bowl, whisk the egg yolks, sugar and cornstarch.
Gradually, add hot milk to egg mixture, whisking constantly until all milk has been added.
Return mixture to pot and stir with a wooden spoon over medium-low heat for about 4 minutes until custard thickens and is able to coat the back of a spoon. Remove from heat and strain.
Let mixture cool to room temperature and then stir in whipping cream. Chill custard completely.
Pour chilled custard into ice-cream maker and freeze according to manufacturer's instructions. In the last few minutes, add walnuts and mix until blended. Scrape soft ice-cream into a container and freeze until firm, for about 4 hours.