1. Combine the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment. Start mixing at a slow speed for 1 minute, then increase to medium speed for another 3 minutes or until creamy.
2. Slowly add the flour and mix until the mixture just comes together. Reduce the speed, add the egg, yolk and vanilla, mixing until combined. Place the dough on a tray, cover with clingfilm and chill in the fridge for at least 1 hour.
3. Roll out the dough between 2 sheets of parchment paper into a rectangle 5mm thick. Place into the freezer while preheating the oven to 160°C, gas mark 4 (do not use a fan setting). Meanwhile, place the ingredients for the lavender sugar in a blender and blitz to a fine powder.
4. Place the dough onto a baking tray and remove the top layer of parchment. Cook for 12 minutes. Remove from the oven and cut into 24 fingers about 10x3cm. Sprinkle over the lavender sugar and return to the oven for 10 minutes until firm and just starting to colour. Remove from the oven and sprinkle with a little lavender. Leave to cool slightly before serving alongside a Heston from Waitrose Fruit Cup.