In a saucepan, heat cream and buttermilk until it steams; then remove from heat. Meanwhile, beat egg yolks with half the maple syrup until pale and fluffy, about 3 minutes. Pour ½ cup of hot cream/buttermilk mixture into egg mixture, whisking as you go; then transfer it all back to the pot to create a loose custard.
Stir in remaining maple syrup. Heat custard on medium setting, stirring continuously, until it reaches 175° on an instant-read thermometer (this is the point at which it will thicken noticeably). Remove from heat, stir in salt, and pour through a strainer into a bowl. Place in refrigerator to chill to 35°–40°, about 6 hours or up to overnight. (You may speed up this process by stirring the custard over a bowl of ice.)
Pour mixture into your ice-cream maker, leaving ¾ inch at the top to allow for expansion, and prepare according to freezer instructions. Add walnuts during final 5 minutes of chilling.