Preheat your oven to 325°F (165°C). Line an 8x8-inch square baking pan with parchment paper, leaving an overhang on two opposite sides. Set aside.

In a medium bowl, whisk together the gluten-free flour, cornstarch, and salt. Set aside.

In a large bowl, using an electric mixer, cream together the butter and granulated sugar until smooth and light in color, about 2-3 minutes.

Add the vanilla extract to the butter mixture and mix to combine.

Gradually add the dry ingredients to the butter mixture, mixing on low speed until the dough comes together. The dough should be slightly crumbly but still hold its shape when pinched.

Press the dough into the prepared pan and smooth the top with a spatula. Poke holes all over the shortbread dough with a fork.

Chill the dough in the pan for 20-30 minutes to let the butter firm up. This will solidify the butter which helps ensure flaky shortbread.

Bake the shortbread at 325°F (165°C) for 30-35 minutes, or until the edges are lightly golden.

Remove the shortbread from the oven and immediately score the shortbread into 12 rectangles, using a bench scraper.

Let the shortbread cool for 15 minutes.

Remove the shortbread from the pan by lifting the parchment and place on a cutting board. Use a sharp knife to cut the shortbread along the lines you already scored.

Let the shortbread cool completely before serving.