Sift flour, sugar, and baking powder, then cut in butter and vanilla until resembles coarse crumbs.
Mix in the egg until relatively even consistency and pat dough onto the bottom and sides of the pie plate.
Pour berries over crust and in a separate bowl mix topping ingredients together. Pour over berries and bake for 1/2 hour and let stand until room temperature (center will be loose).
Meanwhile, prepare the vanilla sauce by putting milk, potato flour, and egg into a sauce pan. (To avoid lumps stir the potato flour in maybe 1/2 cup of milk and then add it to the rest). Heat over medium element just until boiling point while stirring with a wire whisk (this avoids lumps).
Once heated through and slightly thickened, add the sugar and vanilla and continue stirring for another 2 minutes Let the sauce cool and then refrigerate it until cold. This pie is usually served at room temperature with cold vanilla sauce.