Lemon & pistachio Persian squares

175g soft butter

150g caster sugar

2 eggs, preferably free range

1 tsp vanilla extract

Zest 1-2 lemons

175g self-raising flour

For the icing

300g icing sugar

Freshly squeezed juice of 1 lemon

1 tbsp boiling water

For the topping

40g Persian pistachios, chopped

1 tbsp rose petals

30 x 20cm Swiss roll tin, well-greased or lined with parchment paper