In the bowl of a food processor, combine the flour, sugar and salt. Add the butter and pulse for a few seconds at a time, until the butter is the size of peas. Add the egg and water. Pulse again until the dough just begins to form. Add water, as needed. Remove the dough from the food processor and form a disc with your hands.
On a lightly floured work surface, roll out the dough and line a 23-cm (9-inch) pie plate. Refrigerate for 30 minutes.
With the rack in the middle position, preheat the oven to 220 °C (425 °F).
Cover the pastry crust with aluminum foil and fill with dried beans or pie weights. Bake for 10 minutes or until the crust looks dry. Remove the foil and weights and bake for about 10 minutes or until the crust is cooked and golden brown. Allow to cool.
In a saucepan off the heat, dissolve the cornstarch in the water. Add the maple syrup, cream and egg. Stir to combine.
Bring to a gentle boil, stirring constantly with a whisk. Simmer for about one minute. Add the butter and stir until melted. Pour into the crust. Let cool and refrigerate for about 3 hours or until the filling is firm.