In a large bowl, mix together graham crackers, 1/2 cup butter and brown sugar.
In a microwave-safe container, whisk together sugar and cornstarch. Slowly whisk in milk, eggs, zest, juice and 1/2 cup butter. Microwave for 2-3 minutes, just until boiling. Whisk mixture then return to microwave and cook for 1 minutes. Fold in sour cream, spoon mixture into crust. Refrigerate until firm. Serve with dollops of whipped cream.