In a medium saucepan, combine frozen raspberries and granulated sugar over medium heat. Stir continuously for about 5-7 minutes until thickened.

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

Using a mixer, beat unsalted butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add cornstarch-water mix, milk, raspberry syrup, and red food coloring to the butter mixture and mix until combined.

Gradually add dry ingredient mixture to wet mixture, stirring until just combined, then fold in chopped frozen raspberries.

Cover the cookie dough and refrigerate for at least 30 minutes.

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

Scoop tablespoon-sized portions of cookie dough, roll into balls, and roll in granulated sugar.

Bake for 10-12 minutes or until edges are lightly golden brown, then cool cookies on a wire rack.