Prepare the Cookie Dough:

1. Cream the Butter and Sugar: In a large bowl, cream the softened butter and sugar together until light and fluffy.
2. Add the Egg and Vanilla: Beat in the egg and vanilla extract until fully incorporated.
3. Combine the Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.
4. Add Dry Ingredients to Wet Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
5. Fold in the Blackberries: Gently fold the chopped blackberries into the dough. If you’re using whole blackberries, carefully mix them in so they don’t get too squished.
6. Chill the Dough (Optional): For best results, refrigerate the dough for 15-30 minutes to make it easier to scoop and shape.

Bake the Cookies:

7. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
8. Shape the Cookies: Use a cookie scoop or tablespoon to portion out the dough, shaping it into rounded balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.
9. Bake: Bake the cookies for 10-12 minutes, or until the edges are lightly golden but the centers are still soft.
10. Cool: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Make the Glaze (Optional):

11. Prepare the Glaze: In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. If the glaze is too thick, add a bit more milk.
12. Drizzle the Glaze: Drizzle the glaze over the cooled cookies and top with a fresh blackberry for decoration, if desired.