Preheat your oven to 350°F (175°C). Lightly grease your muffin tin or line it with paper liners to prevent sticking.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt.
In a separate large bowl, cream the softened butter with granulated sugar and brown sugar until light and fluffy, using an electric mixer or whisk.
Add the vanilla extract and egg, beating well to combine.
Gradually add the dry ingredients and mix until just incorporated.
Fold in the chocolate chips gently.
In another bowl, combine the brownie mix with the water. Stir until smooth and fully blended, making sure there are no lumps.
Take about a tablespoon of cookie dough and press it into the bottom and up the sides of each muffin cup, forming a little cookie bowl.
Spoon about a tablespoon of brownie batter into the center of each cookie cup without overflowing.
Bake for 18-22 minutes, or until the edges of the cookie dough are golden brown and the brownie centers have set but remain fudgy.
Remove from oven and let cool for at least 10 minutes before carefully removing from the tin.
Transfer to a wire rack to cool completely or serve slightly warm with a scoop of vanilla ice cream.