Start by preheating your oven to 350°F (175°C). Line your 8x8 baking dish with parchment paper so your brownies come out perfectly.
In a saucepan, melt your unsalted butter until it turns a beautiful golden brown and smells nutty – this will add a rich depth of flavor to your brownies!
Pour this browned butter into a large mixing bowl. Add your semisweet chocolate pieces and let it sit for a few seconds before stirring. The chocolate will melt beautifully into the butter.
Now it’s time to add the sugars and salt! Whisk them in until fully combined. Add in the vanilla extract and eggs, and whisk again until smooth and shiny.
Fold in the cocoa powder and flour until your batter comes together into a thick, fudgy mixture.
Transfer the brownie batter into your prepared baking dish, spreading it out evenly. Pop them in the oven and bake for 35-40 minutes. You’ll know they’re ready when a toothpick comes out clean. Let these brownies cool completely once they’re done.
While the brownies are cooling, it’s time to make the topping! In a large pot, melt the butter over medium heat. Once melted, add in your marshmallows, stirring every minute or so until they're smooth and gooey.
Stir in the cocoa puff cereal, coating it evenly with the marshmallow mixture. This is the fun part – the cereal adds the perfect crunch to balance the gooey topping!
Once your marshmallow cocoa puff mixture is ready, pour it over the cooled brownies. Use a spatula to press the mixture evenly into the dish, making sure it’s well spread out.
Let the brownies set at room temperature for about an hour to allow the topping to firm up.