Butter and flour a 9 or 25 cm pie pan. Preheat the oven to 350. Using an electric mixer or by hand, cream the butter and sugar. Add the egg, and blend well. Combine the flour and baking powder and mix them into the wet ingredients to form a soft dough. With flour dusted fingers, pat the sticky dough into the bottom of the buttered and floured pie pan. Push the dough up to cover the sides of the pan. Refrigerate for at least as long as it takes to make the filling. Mix all the filling ingredients, except the blueberries, until smooth. Put the berries into the pie shell and gently pour in the filling so the berries are coated and evenly distributed. Bake for 50 60 minutes, until the crust is browned and the custard has set. Chill well before serving.