In a large bowl, whisk together the gluten free flour, the baking powder and the kosher salt. Set aside.
In a small bowl, using a spoon, mix together the cornstarch and water until thin and watery. It will be thick and tacky at first.
In a large bowl of an electric mixer fitted with the paddle attachment or using a hand held mixer, beat the room temperature butter or vegan baking stick and the ¾ cup sugar until soft and creamy. About 2-3 minutes.
With the mixer on low speed, beat in the cornstarch water mixture. Then beat in the vanilla and milk. The mixture might look slightly chunky and that’s ok.
Still on low speed, beat the gluten free flour mixture into the wet ingredients, a little at a time until just combined.
Once combined, scrape down the sides of the bowl using a spatula.
Pop the bowl of dough in the freezer for 15 minutes.
In the meantime, preheat the oven to 325 degrees Fahrenheit. Line a baking sheet with parchment paper. Set aside.
Take the bowl of dough out of the freezer. Using an ice cream scoop, scoop the dough out into 6 scoops.
Drop the dough ball into the remaining ¼ cup granulated sugar and coat completely.
Make sure to space the cookies at least 3 inches apart on the baking sheet.
Bake the cookies in the middle rack for 13 minutes or until golden brown on the bottoms. Carefully use a cookie spatula to check.
When the cookies come out of the oven, reshape. Immediately use a spatula to push the cookies back into a circle shape or use a large circle cookie cutter. If using a cookie cutter, place it over the cookies and rotate quickly in a circle.
Do not take the cookies off the baking sheet for 5 minutes. If you try, they will fall apart. After 5 minutes, transfer cookies to a cooling rack and enjoy!