Prepare the Cookie Dough:

1. Mix Dry Ingredients: In a medium bowl, whisk together the flour, black cocoa powder, baking soda, and salt. Set aside.
2. Cream the Butter and Sugar: In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
3. Add the Egg and Vanilla: Beat in the egg and vanilla extract until fully combined.
4. Combine with Dry Ingredients: Gradually add the dry ingredients to the butter mixture, mixing until just combined. Add the milk and stir to form a soft dough.
5. Add Chocolate Chips (Optional): If using mini chocolate chips, fold them into the dough for extra chocolate flavor.

Prepare the Cheesecake Filling:

6. Mix Cheesecake Filling: In a separate bowl, beat together the softened cream cheese, powdered sugar, vanilla extract, and sour cream until smooth and creamy.

Assemble the Cookies:

7. Chill the Dough: Scoop tablespoon-sized portions of the cookie dough and roll them into balls. Place them on a baking sheet lined with parchment paper and refrigerate for at least 30 minutes to firm up.
8. Form the Cheesecake Centers: After the dough has chilled, flatten each dough ball slightly and place about 1 tsp of the cheesecake filling in the center. Carefully fold the dough around the filling to form a ball again, sealing the edges.
9. Bake the Cookies: Preheat the oven to 350°F (175°C). Place the filled dough balls on a lined baking sheet and bake for 9-10 minutes, or until the edges are firm but the center is still slightly soft.
10. Cool the Cookies: Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.