Approx 200g plain or flavoured cookies without icing or cream (to make about 2 cups of cookie crumbs)
1/4 cup (1/2 stick) unsalted butter, melted
1 1/2 lbs rhubarb, sliced (about 4 1/2 cups)
3/4 cup coconut sugar or granulated sugar
2 tablespoons all-purpose flour
1/2 teaspoon ground cinnamon
1/4 teaspoon kosher salt
1/2 cup milk
3 egg yolks
1 teaspoon pure vanilla extract
1 cup whipping (35%) cream
2 teaspoons icing sugar
Make the crust: In the bowl of a food processor, process the cookies into fine crumbs. In a medium bowl, mix the crumbs and butter together until combined well. Transfer the mixture to an 8- or 9-inch pie plate. Using clean hands (knuckles work best), press the mixture evenly into the bottom and up the sides of the pan. Bake the crust in a 350º oven until lightly browned, 15 to 18 minutes. Cool completely. Cover and refrigerate for at least 90 minutes.
Make the filling: In a medium saucepan, stir together the rhubarb, sugar, flour, cinnamon, salt, milk, egg yolks and vanilla. Cook over medium heat, stirring often, until the rhubarb is soft but not mushy, about 8 to 10 minutes. Transfer the mixture to a bowl. Allow to cool slightly, then cover and refrigerate until the mixture is chilled, at least 90 minutes.
Assemble the tart: Pour the rhubarb mixture evenly into the crust. Whip the cream and icing sugar until the soft-to-medium peak stage. Spread the whipped cream evenly over the pie. Serve immediately. (Alternately, the pie can be assembled, refrigerated overnight, and topped with whipped cream just before serving.)