In a saucepan, combine frozen raspberries and granulated sugar over medium heat. Stir continuously until the mixture begins to bubble and thicken into a syrup, about 5-7 minutes. Remove from heat and let it cool completely.

In a large mixing bowl, whisk together gluten-free flour, cornstarch, baking powder, and kosher salt. Set this aside.

In another bowl, cream unsalted butter (or vegan baking stick) with 1 ¼ cups of granulated sugar using a mixer for about 3-4 minutes until fluffy.

Add the cornstarch-water mixture, milk, cooled raspberry syrup, and optional red food coloring to the creamed butter and sugar. Mix until smooth.

Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in chopped frozen raspberries gently.

Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Once chilled, scoop out portions of the dough, roll them into balls, and coat in granulated sugar before placing on the baking sheet.

Bake the cookies for 10-12 minutes or until the edges are golden brown. Cool on the baking sheet briefly before transferring to a wire rack.