Roll out the refrigerated pie crust into a 9-inch deep dish pie plate. Prick generously with a fork. Bake for 8-10 minutes or until lightly browned. Remove to a wire rack to cool (and turn off the oven). (These are general instructions for a refrigerated pie crust, read the instructions on yours to be certain!)
In a medium saucepan, whisk together the sugar and cornstarch. Turn on heat to medium and whisk in the grape juice. Bring to a low simmer for 1 minute. Mixture will thicken to the point that it gives resistance when whisked. Remove from the heat.
Whisk the egg in a small bowl. Ladle in 2-3 tablespoons of the hot grape juice mixture while whisking. Whisk well and then add the egg mixture back into the grape juice mixture, whisking well. .
Add the butter, lemon juice and salt, whisking until the butter melts and mixture it smooth and glossy. Mixture will be thick enough that you can draw a line down the center of the pan and it holds
Transfer the filling to the baked pie crust. Allow to cool at room temperature for 1 hour, then cover and place in the refrigerate for a minimum of 3 hours or until fully chilled.
Spread the whipped cream over the grape juice filling and serve.
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