Place the almond flour, sugar, salt, and flour in a large bowl. Add butter and use your fingers to break the butter into the flour until mixture resembles peas.
Whisk egg yolk with 1/4 cup cold buttermilk in a small bowl and then add the egg mixture to the flour mixture. Mix with a wooden spoon, drizzling in more buttermilk as needed 1 tablespoon at a time, until dough just comes together.
Gently knead dough on a lightly floured surface until no dry spots remain, about 1 minute.
Divide dough in half. Shape each piece into a circular disk and place it in the fridge for 20 minutes,
Grease an 20cm pie dish.
Roll out 1 disk of dough onto a lightly floured surface to a 30cm round. Carefully transfer the pastry to the prepared pie dish
Lift up the edges and allow the dough to sink down into the dish. Trim edges to even out pastry if needed, then prick the bottom of the dough with a fork a few times.
Roll out the remaining disk of dough onto a lightly floured surface to a 30cm round . Transfer to a sheet of grease proof paper and set aside.
In a large bowl toss together the blueberries, sugar, corn flour, vanilla extract and butter. Toss well to coat making sure everything is well mixed.
Spoon the filling into the prepared pie dish. Make sure to scrape in all the Place the other pastry round overtop of the blueberry mix. Push the edges of the top pastry into the pie dish and then crimp the edges of the bottom pastry together with the top pastry.
Brush the top with the beaten egg and using a sharp knife, slice four holes to release steam. Cover and chill the pie until crust is firm, at least 1 hour.
Preheat the oven to 180c.
Place pie in oven and a tray and bake until juices are bubbling and crust is golden brown, around 40-50 minutes. Transfer to a wire cake cooling rack and let cool slightly before serving.