In a small saucepan over medium heat, combine frozen raspberries and granulated sugar. Cook while stirring for about 5-7 minutes until the mixture thickens.

In a large mixing bowl, combine gluten-free flour, cornstarch, baking powder, and kosher salt. Whisk until well blended.

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

Add cooled raspberry syrup, milk, and red food coloring to the creamed mixture. Blend on low speed.

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

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

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

Roll tablespoon-sized portions of dough into balls and roll them in sugar before placing them on the lined baking sheet.

Bake for 10-12 minutes until the edges are golden and the centers look set. Cool on the baking sheet, then transfer to a wire rack.