Preheat the oven to 180°C/350°F. Line a muffin tin with cake cases.
To make the lemon curd, melt butter over low heat and add sugar, lemon juice and egg. Stir until smooth. Continue cooking over low heat, stirring constantly for 5-7 minutes. Pour into glass bowl and cover with plastic wrap directly on top of curd. Refrigerate until cold.
For the cake, mix sugar, lemon rind and flour together. Add melted butter, eggs and milk. Beat for 5 minutes. Divide the mixture between the cake cases. Bake for 20-25 minutes or until the cakes spring back when lightly pressed with a finger.
While the cakes are baking, make the syrup by mixing the sugar and lemon juice over a low heat until the sugar has dissolved. Pour over each little cake after removing from the oven and leave to cool in the tins for 5 minutes.
Cut small pieces out of the top of each cake, making a little well. Fill with lemon curd.
To make the meringue, beat egg whites in small bowl with electric mixer until soft peaks form. Add sugar slowly and continue beating until sugar dissolves. Pipe meringue on top of each cake and freeze.
Thaw for at least 30 minutes prior to cooking meringue. Cook in 220°C/420°F oven for 3 minutes or until lightly browned.
These will keep in the freezer for up to a month.