Preheat the Oven: Set your oven to 375°F (190°C) to ensure it’s hot and ready for the pie.
Prepare the Pie Crust: Place one pie crust in the pie dish. If making a homemade crust, roll out the dough and line the dish, trimming the edges.
Make the Goumi Filling: In a bowl, mix goumi berries, sugar, cornstarch, lemon juice, vanilla extract, and cinnamon (if using). Stir gently.
Assemble the Pie: Pour the goumi mixture into the prepared crust. Dot with butter cubes on top.
Top with Second Crust: Place the second pie crust over the filling. Press the edges together and seal. Cut slits in the top crust.
Bake: Place the pie in the oven and bake for 45-50 minutes, until the crust is golden and filling is bubbling. Cover edges with foil if they brown too quickly.
Cool and Serve: Let the pie cool for 1 hour before slicing.