Sift together flour, cornflour, custard powder, and baking powder.

Rub in softened butter.

Make a well in the center, add egg yolk and enough water to form a firm dough.

Knead lightly.

Cover and refrigerate for at least 30 minutes.

Preheat oven to 190 degrees C/375 degrees F.

Boil potatoes until just tender.

Heat olive oil and butter in a large pan.

Cook onion, carrot, and celery until softened.

Add chopped chicken and herbs.

Add cooked potatoes, corn, shallots, and seasoning.

Mix gently.

Add water to stock and cornflour, mix, and stir into the filling.

Let thicken slightly.

Remove from heat.

Divide pastry into two halves.

Roll out the first half on a floured surface.

Make it large enough to cover the pie plate with an overhang.

Fill the pie with the cooked filling.

Brush egg white around the rim of the pastry.

Roll out the second half of pastry and cover the pie.

Press edges together firmly and trim.

Decorate the pie with any spare pastry.

Brush the top with egg white and sprinkle with sesame seeds.

Cut an X in the top to allow steam to escape.

Bake for 25-35 minutes until golden brown.

Serve immediately.