Preheat the oven at 160C/325/Gas 3.
Cream the butter and sugars together in a bowl with a handheld electric mixer until light and fluffy and pale.
Stir the ground almonds, flour and orange zest into the mixture. Stir until thoroughly mixed. If the room is warm and the mix feels too soft, wrap in cling film and set aside to rest in the fridge for 10 minutes.
Divide the mixture into eighteen equal pieces and roll each into a ball. On a floured surface, flatten each ball to a circle 1cm/½in thick with a fork or with a cookie stamp.
Place them on a non-stick baking sheet at least 2.5cm/1in apart.
Bake for 20-25 minutes, until the biscuits are barely coloured.
Remove from the oven and leave to cool for 10 minutes before transferring to a wire rack to cool completely.
When cool, sprinkle the biscuits with the additional lavender sugar before putting them onto a serving plate.