Butterscotch Pie Supreme

Mix together brown sugar, butter, corn syrup, and 1⁄4 cup water in a heavy-bottomed medium saucepan. Cook, stirring constantly, over medium heat until mixture reaches 250° on a candy thermometer, about 15 minutes. Immediately lower heat to keep the temperature at 250°, keeping butterscotch warm over low heat.

Mix together flour and cornstarch in a medium bowl. Add 1⁄2 cup milk, whisking until smooth. Whisk in egg yolks, then set aside. Combine sugar, salt, and remaining 1 1⁄4 cups milk in a heavy saucepan and bring just to a boil over medium heat, about 10 minutes. Whisk 1⁄2 cup hot milk mixture into yolk mixture, then whisk yolk mixture into remaining hot milk. Cook over low heat, whisking constantly, until thick and boiling, about 1 minute. Boil for 1 minute more, remove from heat, and stir in vanilla.

Immediately mix together warm butterscotch and custard mixture. Pour into baked pie shell. Allow to cool to room temperature, then refrigerate until set, about 2 hours. Just before serving, top pie with whipped cream, leaving butterscotch exposed in center. Sprinkle with walnuts.