1 (9-inch) baked pie crust (recipe below)
8 tablespoons unsalted butter (1 stick)
1 cup dark brown sugar (packed)
1 cup boiling water
3 tablespoons cornstarch or arrowroot
2 tablespoon unbleached all-purpose flour
½ teaspoon salt
1⅔ cups milk
3 egg yolks
1 teaspoon pure vanilla extract

For the topping:
1 cup heavy whipping cream
2 tablespoons confectioners sugar
½ cup roasted, salted cashews, chopped