Beat softened butter and powdered sugar together in a large bowl until light and creamy, about 3 minutes. Add vanilla extract and mix to combine.
Whisk together gluten free flour, cornstarch, and salt. Add the dry ingredients to the butter mixture and stir until a smooth dough forms.
Turn dough out onto a sheet of plastic wrap. Shape into a log about 2 inches in diameter, wrap tightly, and refrigerate for at least 1 hour until firm.
Preheat oven to 325F and line a baking sheet with parchment paper. Slice the dough log into 1/4-inch thick rounds and place on the baking sheet 1 inch apart. Sprinkle each cookie with a tiny pinch of flaky sea salt.
Bake for 14 to 16 minutes until the edges are just barely golden but the centers are still pale. Let cool on the baking sheet for 10 minutes before transferring to a wire rack. Cookies will firm up and become perfectly crumbly as they cool.