Combine MILKMAID and eggs in a bowl over simmering water. Stir until mixture coats the back of a wooden spoon. Remove from heat, whisk in butter, a few cubes at a time add lemon zest and juice, mixing well; allow to cool.
Spoon lemon filling into pastry case and refrigerate until set.
Sift sugar over tart, cook under a hot grill until golden. Refrigerate until cold.
Sift flour and icing sugar into a bowl; rub in butter until mixture resembles breadcrumbs. Roll out to fit the prepared tin, refrigerate 30 minutes. Prick base all over with a fork. Bake 15 minutes or until light gold. Remove from tin and cool.