Preheat the oven to 350°F/180°C and line a pie pan with parchment paper.
Mix the softened butter, sugar, vanilla, and salt. Add the the flour progressively, until you get a crumbly dough. Press the dough evenly at the bottom of your lined pie pan and up the sides. Poke the crust with a fork then chill in the fridge for about 30 min.
Bake the pie crust until golden, about 25 min. Meanwhile, prepare the filling. Mix the cream, milk, sugar and salt in a saucepan and bring to simmer. Remove from the heat and add the chocolate broken into pieces. Gently stir until fully melted and uniform. Whisk in the egg then pour the filling in the crust.
Decrease the oven heat to 300°F/150°C before baking the pie again for about 15 min. The filling should be set at the edges but still wiggly in the middle. Once you reach that stage, take the pie out and let it cool at room temperature. Sprinkle the salt flakes on before the filling fully sets, then carefully unmold the pie.
This pie is on the rich, bitter side - you’ll need to pair it with an equally intense beverage. I recommend a fruit-forward wine such as Merlot or an oolong.
If you have smoked sea salt on hand, you can also use that to top the pie for some extra flavor. If you’re in a fully decadent mood, or if the cocoa proves too bitter to handle, a dollop of crème fraîche or whipped cream on top of your slice will help.