To make gluten-free pistachio shortbread cookies, start by gathering all of the necessary ingredients. Measure out the pistachio nuts and crush them in a food processor. Set them aside for later use.

Next, add the butter and sugar to a mixing bowl. Attach a paddle attachment to the mixer and cream the butter and sugar together until the mixture is creamy and the sugar has dissolved slightly.

Once the butter and sugar are creamed, add an egg, vanilla extract, and a pinch of sea salt to the mixture. Mix everything together until it is well combined.

Now it is time to add the gluten-free flour, crushed pistachios, and baking powder to the mixing bowl. Mix everything together on a low speed until a dough forms.

Transfer the dough from the mixing bowl onto a lightly floured surface.

Knead the dough into a ball and roll it out until it is about ¼-inch thick. Be sure to sprinkle more gluten-free flour on the dough as needed to prevent sticking.

Using a cookie cutter with a diameter of 2.5 inches, cut out the cookies from the rolled-out dough.

Carefully transfer the cookies onto a baking sheet lined with parchment paper using a chef's spatula.

Note: If the cookies are still a bit warm, they may be fragile; handle them gently. Use the chef spatula to carefully transfer the cookies onto a cooling rack.

Now it is time to make the chocolate drizzle. In a bowl placed over a hot water bath, melt the chocolate chips and milk (or almond milk) together. Stir until the chocolate is completely melted and smooth. Set the chocolate aside to cool slightly before using it.

To decorate the cookies, pour the chocolate ganache into a sandwich-size Ziploc bag or a chocolate dispenser bottle.

Drizzle the chocolate on top of each cookie and then sprinkle crushed pistachio nuts over the chocolate drizzle.

Allow the chocolate to set before serving or storing the cookies.