Preheat the oven to 350 degrees Fahrenheit. Prepare a baking sheet with parchment paper, or spray with non-stick spray.
In a large bowl, make the brownie mix based on the directions on the box.
Bake in the oven for 15 minutes or until a toothpick inserted comes out clean. Because the bars are going to be thin, the bake time will be much shorter than the directions say on the box.
Remove the brownies from the oven and coat evenly with the mini marshmallows. Place the brownies back in the oven for 3-5 minutes until the marshmallows have puffed and become soft.
Remove from the oven and let cool.
In a saucepan make the chocolate sauce by melting the butter, milk, salt and unsweetened cocoa powder over medium heat. Once melted, and combined with no lumps, remove from the heat.
Add the vanilla extract and powdered sugar, whisking together until smooth and creamy. If the sauce is a bit thick for drizzling over the bars, then add a little more milk to thin it out.
Pour the chocolate sauce diagonally over the brownie bars and top with the chopped pecans.
Notes
These bars are easiest to cut when completely cooled.
Store brownies in an airtight container at room temperature.