Combine flour, butter, sugar, vanilla extract, baking powder and eggs; mix well. Knead quickly until a smooth dough forms. Divide the dough by removing 2/3's of it and chilling this portion for 30 minutes. Freeze the remaining dough (you will use this as the topping).
Preheat the oven to 180 C / Gas 4. Lightly grease a round tart tin.
Combine rhubarb and sugar in a saucepan; pour in a little water and simmer for 6 to 8 minutes until it has broken down a little. Set aside to cool.
To make the meringue: whip the egg whites and sugar together until stiff peaks form.
Roll out the chilled portion of pastry and line the bottom and sides of the tin. Pour in rhubarb mixture, then spread the meringue on top. Grate the frozen pastry on top to form a crumble-like topping.