Photograph: Maja Smend
Grind the lavender sugar with the extra lavender pieces using a pestle and mortar.
Tip the ground sugar into the bowl of a food processor and beat with the butter until pale, light and very fluffy. Mix in the vanilla extract. Sift in the flour and cornflour, and mix until smooth and thoroughly combined.
Spoon the dough into a piping bag fitted with a 1cm star nozzle and pipe 30 x 4cm stars on to a baking tray lined with baking paper. Chill in the fridge for at least 45 minutes.
Preheat the oven to 180°C, fan 160°C, gas 4. Bake on the middle shelf of the preheated oven for 14-16 minutes until pale golden. Cool on the baking sheets for 5 minutes, then transfer the shortbreads to a wire rack to cool.
Whisk the icing sugar and butter together until pale and creamy. Mix in the orange zest and juice, then spoon into a piping bag and pipe on to half of the biscuits. Sandwich together and dust with icing sugar.
Get ahead: bake the shortbreads up to a day ahead, cool and store in an airtight container. The unfilled shortbreads can also be frozen. Once assembled with the filling, they're best eaten on the day.
Kitchen secret: for St Clements (orange and lemon) shortbreads, use caster sugar and add the finely grated zest of 1 lemon.