Brush a 23cm fluted flan tin with melted butter or oil. Place flour, baking powder and butter in food processor.
Process for 30 seconds using the pulse action. Add water and process a further 30 seconds or until the mixture comes together.
Turn onto a lightly floured surface and press together until smooth. Roll out the pastry between 2 sheets of plastic wrap, to cover base and side of the tin.
Ease pastry into the tin and trim edges. Cover with plastic wrap and refrigerate 20 minutes.
Preheat oven to moderate 180°C/350°F Cut a sheet of greaseproof paper to cover pastry-lined tin.
Spread a layer of dried beans or rice evenly over the greaseproof paper. Bake for 10 minutes, then remove from the oven and discard paper and beans. Return the crust to the oven and bake for a further 10 minutes or until it is lightly golden. Allow to cool completely.
To make filling: Combine flour, cornflour and sugar in a medium pan. Gradually stir in lemon juice and water and blend until smooth. Stir over a low heat until mixture boils and thickens.
Remove from heat, stir in butter and egg yolks. Return to low heat and stir until butter has melted. Cover with plastic wrap and cool completely. Place in prepared pastry.
To make topping: Beat egg whites in a large, clean, dry mixing bowl with electric beaters until stiff peaks form. Add sugar gradually, beating constantly until mixture is glossy and thick.
Preheat oven to moderately hot 210°C.
Spread the filling over the pastry base and then spoon the meringue on top of the filling, spreading it to the edges of the pie to seal. Peak the meringue using a flat bladed knife.
Bake for 5 minutes or until meringue is lightly browned on top.
NOTE: For best baking results make the pastry and the filling the day before and store separately. The meringue only needs to cook for about 5 minutes - enough for it to brown lightly. If left in the oven too long, the pie filling will warm and this makes the pie difficult to cut.