Pastry : Combine sugar and one egg in a food processor until frothy. Add butter, flour, baking powder, vanilla and salt. Process until mixture clumps together in a ball around the blade. Wrap dough in baking paper or cling film and chill for 30 minutes
Grease a 25cm loose bottomed, high-sided quiche/pie tin. Press pastry dough into the base and up the sides of the prepared tin. Chill again for 30 minutes. Preheat oven to 180°C conventional.
Place a sheet of baking paper on top of pastry and fill with dry rice or baking weights. Blind bake for 20 minutes, until golden and crisp around the edges. Return to oven for 4 minutes, until browned all over. Whisk remaining egg and brush over pastry base while still hot.
Filling : While pastry base cooks, make filling. Place chocolate and butter in a small heavy-based saucepan and heat on low, stirring, until melted and smooth. Remove from heat.
Using an electric beater, whisk sugar, eggs and egg yolks until pale and creamy (about 5 minutes). Stir in the melted chocolate mixture until well combined, then gently fold in the flour.
Pour the chocolate filling into the cooked pastry case. Bake for 20 minutes.
Meringue Topping : Meanwhile, using a powerful electric beater or stand mixer, beat egg whites until soft peaks form. Beat in sugar, one tablespoon at a time, until all incorporated and mixture is thick, glossy and no longer grainy.
Spoon meringue on top of filling, starting with the outside edges, then piling the remaining meringue in the centre. Try using a fork to create a nice pattern.
Bake pie for 13-15 minutes, until meringue is firm and golden. Cool in tin for about 2 hours (or longer, if preferred). Serve at room temperature with whipped cream or ice cream.