Place the egg yolks in a small mixing bowl. Whisk lightly. Set aside.
In a medium, heavy bottom good saucepan combine the cornstarch, flour, and salt. Whisk in the milk until blended. Set aside.
In a large light-colored skillet melt the butter over medium/low heat and cook stirring constantly until it turns nutty brown in color. (see note) Gradually add the dark brown sugar. Cook, stirring constantly, until it is melted, about 5 minutes. Carefully add 1 cup of boiling water. This is like making caramel as the mixture will sputter and spit.
Add the brown sugar mixture to the saucepan with the milk and flour. Heat over medium-low heat, stirring constantly until it begins to boil. This could take 10-15 minutes. Keep stirring.
Once the mixture has thickened, remove from the heat. Drizzle spoonfuls of the hot butterscotch into the egg yolks while whisking constantly. Repeat until you have added 1/3 of the butterscotch to the egg yolks.
Pour the egg mixture into the saucepan with the remaining butterscotch and return to the heat.
Cook, stirring constantly, until the mixtures begins to boil. Cook for another minute then remove from the heat and stir in the vanilla.
Pour the custard into the pre-baked pie shell. Cool then refrigerate until ready to serve.
To make the topping whisk the heavy cream until soft peaks form. Add the confectioners sugar a little at a time and whisk until thickened. Dollop the whipped cream on top of the pie then add chopped cashews.