Bring milk, whipping cream, 1 cup sugar, and vanilla beans to a boil in a large, heavy saucepan over medium heat, whisking constantly. Remove from heat.
Whisk together egg yolks and remaining 1 cup sugar in a large bowl. Gradually add one-fourth hot mixture into yolk mixture; whisk back into remaining hot mixture.
Cook over low heat, whisking constantly, 10 to 15 minutes or until mixture thickens.
Pour mixture through a fine wire-mesh strainer into a large bowl, discarding vanilla beans. Immediately set bowl in ice, and let stand, stirring constantly, until cool.
Stir in walnuts.
Pour mixture into the freezer container of a 1-gallon electric freezer. Freeze according to manufacturer's instructions.
Pack freezer with additional ice and rock salt, and let stand 1 hour before serving.
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