In a food processor, combine the flour, powdered sugar, salt, lemon zest, and dried lavender. Pulse until evenly mixed.

With the processor running, add the cold butter one tablespoon at a time until the mixture becomes crumbly.

Add the vanilla extract and lemon juice, then process just until the dough comes together.

Turn the dough onto a lightly floured surface and gently shape it into a disk.

Line an 8- or 9-inch square baking pan with parchment paper and press the dough into an even layer.

Score the dough into squares using a sharp knife or bench scraper.

Prick the dough all over with a toothpick or skewer to prevent puffing.

Refrigerate for 30 minutes.

Preheat the oven to 300°F (150°C).

Bake the Iced Lemon Lavender Shortbread Cookies for about 45 minutes, or until lightly golden.

Remove from the oven and immediately re-cut along the scored lines.

Allow the cookies to cool completely in the pan.

In a small bowl, whisk together the powdered sugar, meringue powder, and lemon juice until smooth.

Dip the tops of the cooled cookies into the icing and gently shake off any excess.

Before the icing sets, sprinkle with dried lavender or fresh lemon zest.

Allow the icing to dry completely for 4–6 hours before storing.