Pre-heat the oven to 180 C / Gas 4 and line a cupcake tray with paper cases.
Starting with the lemon curd, place the sugar and lemon juice in a large bowl, add the eggs and butter and start whisking. Set the bowl over a pan of simmering water. Continue to whisk until the mixture has thickened. Measure out 75g for the cupcakes and store the rest in a sterilised jar.
For the cupcakes: Cream the butter, vanilla seeds and sugar in a bowl, and whisk until pale and fluffy. Add eggs one at a time, and beat until well mixed. Fold in the flour and lemon zest
Pipe a layer of cake mixture into the bottom of the paper cases. Add a teaspoon of lemon curd then cover with more mixture.
Bake cupcakes in the oven for 15 to 20 minutes or until pale and golden brown and they spring back when lightly pressed. Remove from oven and cool completely.
For the meringue topping, make a sugar syrup by dissolving the sugar in the water over a gentle heat until it reaches 118 C on a sugar thermometer. Remove from heat.
Whisk the egg whites in a clean bowl until soft peaks and doubled in size. Gradually add the hot sugar syrup in a very thin stream whilst continuing to whisk. Make sure the syrup doesn't touch the blades of the mixer. Whisk continuously until the mixture is thick and glossy and has cooled down. Check by feeling the side of the mixing bowl.
Pipe mixture onto the cooled cupcakes. Using a blowtorch lightly brown the top of the meringue. Decorate with lemon zest and a dusting of icing sugar, if desired.
Lemon curd tip
Make sure the bowl does not come into contact with the simmering water, this will heat the mixture too quickly and will cook the eggs.