In the 1930s, Rudolph Boysen was looking for a place to grow a new berry variety when he met the Knotts, who were struggling to make ends meet on their California farm. They teamed up, and soon the Knott family was growing boysenberries, which taste like a cross between a blackberry and a raspberry. The Knotts' kitchen became the Chicken Dinner Restaurant, and the rest of the property transformed into the theme park we know today. The park honors its history with treats like boysenberry sherbet, cotton candy and this double-crust pie, still made from the Knotts' original recipe.
Make the crust: Combine the flour, granulated sugar and salt in a food processor; pulse to combine. Add the shortening and pulse a few times until the mixture looks like coarse meal. Add the butter and pulse a few times until pea-size pieces form. Pulse in the vinegar, then pulse in the ice water until the dough starts to come together but is still clumpy (you do not want it to form a ball). Divide the dough in half and form into 2 disks. Wrap each disk in plastic wrap and refrigerate at least 1 hour or overnight.
On a lightly floured surface, roll out each disk of dough into a very thin 12-inch round (you can also roll the dough between 2 sheets of lightly floured parchment paper). If the dough is too soft, return to the refrigerator for about 20 minutes. Press 1 round of dough into a 9-inch pie plate. Trim the excess dough, leaving a 1/2-inch overhang.
Make the filling: Combine the boysenberries and liquid, the granulated sugar, cornstarch and salt in a large bowl. Stir until well combined.
Pour the berry filling into the prepared pie crust. Place the second round of dough over the pie and trim the excess to match the bottom layer. Fold the overhanging dough under itself and crimp as desired. Cut a few slits in the top crust. Refrigerate at least 1 hour before baking.
Put a baking sheet on the middle oven rack and preheat to 425 degrees F. Brush the pie with heavy cream and sprinkle with coarse sugar. Put the pie on the hot baking sheet and reduce the oven temperature to 375 degrees F. Bake until the crust is deep golden brown and the filling is bubbling, 60 to 75 minutes. Transfer the pie to a rack and let cool at least 4 hours before slicing.