To prepare ice cream, combine milk and half-and-half in a medium, heavy saucepan, and bring to a boil. Remove from heat.
Combine maple syrup and egg yolks, stirring well with a whisk. Gradually add half of the hot milk mixture to yolk mixture, stirring constantly with a whisk. Return milk mixture to pan. Cook over medium-low heat for 2 minutes or until a thermometer registers 160°, stirring constantly. Place pan in a large ice-filled bowl. Cool completely, stirring occasionally. Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions.
To prepare buttercrunch, line a baking sheet with foil; coat foil with cooking spray. Combine sugar, corn syrup, and 1 tablespoon water in a small saucepan over medium-high heat; bring to a boil, stirring occasionally until sugar dissolves. Cook about 8 minutes or until golden (do not stir). Remove from heat. Stir in walnuts, butter, and salt. Quickly spread buttercrunch in a thin layer on prepared baking sheet. Cool completely. Break buttercrunch into pieces. Place buttercrunch pieces in a food processor; pulse 4 times or until coarsely ground. Spoon ice cream into a freezer-safe container; stir in buttercrunch. Cover and freeze 1 hour or until firm.