Preheat the oven to 350°F (177°C). Line an 8x8-inch baking pan with aluminum foil, leaving some overhang on the sides for easy removal. Grease the foil and set aside.
Add the melted butter to a large mixing bowl. Whisk in the cocoa, eggs, sugar, and vanilla until combined.
Stir in the flour and salt with a rubber spatula or wooden spoon until incorporated.
Fold in the walnuts. Pour the batter evenly into the prepared baking pan.
Bake for 20–25 minutes until set and a toothpick inserted in the center comes out with only a few moist crumbs.
Remove from oven and allow to cool completely in the pan set on a wire rack.
Once cooled, lift the brownies out of the pan using the foil overhang on the sides and cut into squares.
Notes
The brownies will stay fresh in an airtight container at room temperature for up to 1 week or in the freezer for up to 3 months.