Preheat the oven to 350 degrees. Line an 8x8 baking dish with parchment paper, then lightly spray with non-stick spray and set aside.
In a large bowl, prepare the brownie batter per the box (or use one of the scratch recipes I linked above)
Your favorite box mix + ingredients required
Melt the butter in either a microwave safe dish, or in a small saucepan over medium heat. Once melted, stir in the sugar until fully combined. Then, stir in the crushed pretzels.
1 cup pretzels, 4 Tablespoons butter, 2 Tablespoons sugar
Press the pretzel mixture into the bottom of your prepared baking dish in an even layer.
Bake the crust for 10 minutes.
Remove the crust from the oven and pour the prepared brownie batter over the top.
Bake brownies for 25 minutes or until toothpick inserted in the center comes out clean.