Elderflower meringue pie

For the pastry:

90g butter, chilled and cubed 

150g flour

35g caster sugar

1 egg yolk

1 tsp vanilla extract

1 pinch of salt

For the filling:

90g golden caster sugar

2 tsp cornflour

zest of 2 lemons and 140ml of lemon juice, strained

40ml elderflower cordial added to 145ml of water

85g butter, cubed

3 egg yolks and 1 egg, lightly beaten together

For the meringue:

4 egg whites

190g caster sugar

2 tsp corn flour