In a food processor, with the standard blade in place, grind the pistachios until fine. Remove the pistachios to a small bowl.

In the bowl of the food processor, add the butter, confectioner’s sugar, granulated sugar and lemon zest. Process the mixture until light and fluffy, about 1 minute.

Scrape down the sides and bottom. Add the almond extract, vanilla extract and lemon extract (if using). Pulse 5 times to blend the extracts in.

Scrape, scrape, scrape. Add the almond flour, ground pistachios, oat flour and salt. Pulse the mixture about 10 times, scrape down the sides and continue pulsing until the mixture is in small crumbles.

Pour the mixture onto a pastry board or clean counter. Divide the mixture in two. Bring the crumbles together to form a dough and roll into a 2-inch (5 cm) wide log.

Roll the log in granulated sugar and wrap in parchment paper or plastic wrap. Repeat with the other half of the dough and refrigerate both logs for at least 2 hours, or overnight if possible.

When ready to bake, heat the oven to 350°F/180°C. Line two baking sheets with parchment paper.

Slice the logs of dough into ½-inch discs. Lay the cookies on the baking sheet with half-inch space between each.

Bake for 12 -14 minutes until the edges turn light golden. If you touch the middle of the cookie and you leave an indent, the cookies are not yet done.

While the shortbread are still warm, sift over confectioner’s sugar.