In a medium microwave-safe bowl, heat the frozen blueberries in 20-second intervals, stirring after each, until they become soft and jam-like. This should take about 2 to 2.5 minutes. Allow them to cool, as they will thicken further.

In a large bowl, combine the gluten-free flour, cornstarch, baking powder, and kosher salt, whisking them together well. Set this mixture aside.

In a small bowl, mix the remaining cornstarch with water until it reaches a thin, watery consistency. It may be thick and tacky at first.

Using an electric mixer with a paddle attachment, beat the unsalted butter or vegan baking stick with the granulated sugar until the mixture is soft and creamy, which should take about 2 to 3 minutes.

Gradually add the cornstarch-water mixture, milk, and 3 tablespoons of the cooled blueberry mixture into the creamed butter and sugar, mixing until combined. The mixture will appear magenta and chunky.

Slowly incorporate the gluten-free flour mixture into the wet ingredients, mixing just until combined. The dough will be very sticky and lavender in color.

Cover the bowl with plastic wrap and refrigerate the dough for at least 2.5 hours. This step is crucial to prevent the cookies from spreading too much. If you chill the dough overnight, it will turn blue with flecks.

After chilling, preheat your oven to 325 degrees Fahrenheit and line a baking sheet with parchment paper.

Using a large cookie scoop, portion out 12 dough balls. If the dough sticks to the scoop, rinse it halfway through to remove any residue.

Roll each dough ball in the reserved granulated sugar to coat them completely, then place them about 3-4 inches apart on the prepared baking sheet.

Bake the cookies in the preheated oven for 15 minutes. If you have extra dough balls, store them in the fridge while the first batch bakes. I recommend baking only 6 at a time.

Once out of the oven, immediately reshape the cookies into a circular form using a spatula. Let them cool on the baking sheet for 5 minutes before transferring them to a cooling rack. This step is important to prevent the cookies from breaking apart.

Repeat the baking process with the remaining dough balls.