01 - Combine frozen raspberries and granulated sugar in a saucepan over medium heat. Stir constantly and cook until the berries release their juices and become syrupy, about 10 minutes. The mixture will remain slightly chunky.
02 - Set a mesh strainer over a bowl and pour the raspberry mixture through. Allow the strained syrup to cool; you should have approximately 2–3 tablespoons.
03 - Roughly chop half a cup of frozen raspberries into small pieces. Take care not to overprocess, avoiding excess juice. Return the pieces to the freezer until needed.
04 - In a large bowl, whisk together the gluten-free flour, baking powder, 1.5 tablespoons of cornstarch, and kosher salt. Set aside.
05 - In a small bowl, stir 2 heaping tablespoons cornstarch with 3 tablespoons water until smooth and thin.
06 - In the bowl of a mixer fitted with a paddle attachment, beat the room temperature butter and 1.25 cups granulated sugar until soft and creamy, about 2–3 minutes.
07 - Gradually beat in the cornstarch slurry, milk, and cooled raspberry syrup to the creamed butter and sugar. The mixture will develop a pink, chunky appearance.
08 - Add the dry ingredient mixture to the wet, blending a little at a time until just incorporated. Do not overmix.
09 - If desired, blend in 3 drops of red food coloring for a vibrant hue.
10 - Gently fold frozen chopped raspberries into the dough by hand, creating a marbled pink effect. Mix minimally to avoid excess moisture.
11 - Cover the bowl with plastic wrap and freeze for at least 1 hour. This step is essential for maintaining shape during baking.
12 - After chilling, preheat oven to 325°F (163°C) and line a baking sheet with parchment paper.
13 - Scoop dough into 12 large balls using an ice cream or large cookie scoop. Roll each in the reserved 2 tablespoons granulated sugar to completely coat.
14 - Place dough balls at least 3–4 inches apart on the prepared baking sheet. Bake at 325°F (163°C) for 15 minutes. Keep remaining dough in the freezer between batches.
15 - Immediately after baking, use a spatula to reshape cookies into circles. Allow to cool on the sheet for 5 minutes, then transfer to a wire rack.