Take the butter to room temperature well in advance. When the butter is soft enough, whisk it into a light foam with the sugar. Add the egg whites and whisk.
Mix the dry ingredients together first. Then knead into a dough with the butter mixture. Add flour until the dough comes away from your fingers. Spread evenly into the pie dish.
Vanilla custard
Whisk the milk and egg yolk in a saucepan and heat gently. When the sauce starts to bubble, remove it from the heat. Stir in the remaining ingredients and whisk well. The sauce should be quite thick, adding more potato flour if necessary. Pour the custard over the bottom of the pie dish.
Combine blueberries, potato flour and sugar. Spread the mixture over the custard.
Bake the blueberry pie, covered with foil, in a 200C oven for about 45 minutes. Allow to cool for a while and bake! With a glass of milk it tastes like mammon!