Preheat the oven to 180°C. Have the pie dough (par-baked or not) ready nearby, placed on a baking tray to catch any drips.
In a medium bowl, whisk together the melted butter, maple syrup, light brown sugar & the cornflour until combined.
In another medium bowl, whisk together the eggs, yolk, cream & vanilla – ensuring everything is thoroughly combined.
Gradually pour the cream mix into the butter mix & whisk until just combined. Brush the edges of the pie shell with egg wash.
Pour the filling into the pie shell (to about 1-inch down from the top) & bake in the preheated oven for 45-60 minutes, until the edges are puffed & the centre jiggles only slightly when shook. Place the pie dish onto a wire rack to cool completely before slicing – about 4-6 hours.
Once fully cooled, slice into wedges & sprinkle generously with flaky sea salt. Enjoy!