Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together the gluten free flour, baking powder, and kosher salt.
Cream together the unsalted butter with granulated sugar until light and fluffy.
Mix in the cornstarch-water mixture, milk, lemon juice, and lemon zest into your butter-sugar blend.
Gently fold in the chopped frozen raspberries into your cookie dough.
Combine 3 tablespoons of granulated sugar with 1 teaspoon of lemon zest and roll your cookie dough balls in this mixture.
Place the cookies into your preheated oven and bake for about 15 minutes or until set at the edges but still slightly soft in the center.