2 1/2 cups fresh blackberries 1 1/2 cup granulated sugar 6 tablespoons cornstarch 1/4 teaspoon kosher salt 2 tablespoons unsalted butter 4 egg yolks, beaten 1 prebaked pie crust (9-inch size), thoroughly cooled 1/2 cup chopped walnut pieces 1 cup chilled whipping cream 2 tablespoons firmly packed brown sugar 1 tablespoon bourbon
Use a wooden spoon to press the berries through a sieve or strainer to remove the seeds. Discard the seeds and put the puree in a heavy saucepan over medium-low heat.
Sift together the sugar, cornstarch and salt. Add them to the puree and stir. Add the butter. Continue to stir until the sugar and butter are melted and the mixture is steamy but not boiling.
Whisk several spoonsful of the hot berry puree into the beaten egg yolks to temper them. Then whisk the yolks into the puree in the pan. Continue to stir and cook until the mixture boils. Reduce the heat and simmer 1 minute.
Pour the berry mixture into the prebaked pie crust. Cool it to room temperature.
Sprinkle the walnuts over the top. Using the back of a large spoon or the palm of your hand, very lightly press them into the custard just enough to set. Cover tightly with plastic wrap and refrigerate for at least 1 hour before serving.
When you're ready to serve, whip the cream with the brown sugar and bourbon until light peaks form. Serve slices of pie with a generous dollop of bourbon whipped cream on each.