Preheat the oven 170°C fan. Place the flour, salt, butter and sugar in the bowl of a food processor; process until it resembles fine breadcrumbs. Add the lemon zest, lavender and egg yolk; pulse until it’s just combined. Turn the dough out onto the work surface and form into flat square; place in a freezer bag and transfer to the fridge to chill for 30 minutes.
Place a sheet of parchment paper on the work surface. Place half of the chilled dough on the paper; top with another sheet of paper. Roll out the dough between the sheets until it is approximately 3mm-thick. Using a round fluted cookie cutter, cut out the dough and transfer to prepared baking sheets. Roll out the scraps, and repeat. Repeat process with the remaining dough.
Bake in the oven for 12-15 minutes. or until they’re pale golden. Transfer to a wire rack to cool. Dust with caster or icing sugar and add a few lavender petals, if desired.