01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a mixing bowl, cream together softened butter and granulated sugar until light and fluffy.
03 - Beat in the egg and vanilla extract until fully combined.
04 - In a separate bowl, whisk together gluten-free flour, baking powder, and salt.
05 - Gradually add dry ingredients to the wet mixture and mix until a soft dough forms.
06 - Gently toss halved blackberries with 1 tablespoon sugar and lemon zest in a small bowl.
07 - Combine gluten-free flour, brown sugar, cinnamon, and salt. Add cold, cubed butter and rub in with fingertips until mixture resembles coarse crumbs.
08 - Scoop 2-tablespoon portions of dough onto prepared baking sheets, spacing about 2 inches apart.
09 - Gently press a few blackberry pieces into the center of each cookie.
10 - Sprinkle a generous spoonful of crumble topping over each cookie, pressing lightly to adhere.
11 - Bake for 16 to 18 minutes until edges are golden and crumble is crisp.
12 - Let cookies cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.