Preheat the oven to 350°F and line a baking sheet with parchment paper for easy cleanup.

In a mixing bowl, prepare the brownie mix according to the instructions on the box, then gently fold in the chopped pecans.

Spread the brownie batter onto the prepared baking sheet, ensuring an even layer, and bake in the preheated oven for approximately 15 minutes, or until a toothpick inserted into the center comes out clean. Keep in mind that the bake time may be shorter than indicated on the box due to the thinner layer of batter.

Once baked, promptly remove the brownies from the oven and evenly distribute the mini marshmallows over the top. Return the pan to the oven for an additional 3-5 minutes, or until the marshmallows have puffed up and become soft and slightly golden.

Allow the brownies to cool in the pan before proceeding.

While the brownies are cooling, prepare the chocolate sauce by melting the butter, milk, salt, and unsweetened cocoa powder together in a saucepan over medium heat. Stir continuously until fully melted and combined, ensuring there are no lumps.

Remove the saucepan from the heat and stir in the vanilla and powdered sugar until smooth and creamy. If the sauce is too thick for drizzling, simply add a splash more milk to achieve the desired consistency.

Once the brownies have cooled, drizzle the chocolate sauce diagonally over the top, covering them evenly.

Finally, cut the brownies into bars, serve, and enjoy the delicious combination of fudgy brownies, crunchy pecans, gooey marshmallows, and rich chocolate sauce!