Melt the butter and chopped chocolate together in a large microwave-safe bowl by heating in 30 second increments, stirring between each burst of heat, until melted and smooth. Allow the mixture to cool slightly for 10 minutes.
1 cup salted butter, 8 ounces semi-sweet chocolate
Heat oven to 350°F. Line a 9x13 baking pan with a parchment paper sling.
Whisk both sugars into the cooled chocolate and butter mixture. Add the eggs, one at a time, whisking after each addition until smooth. Add the vanilla, then add the salt, flour, and cocoa powder at the same time and gently fold them in using a rubber spatula or wooden spoon, until no streaks of flour or cocoa powder remain.
1 1/2 cups granulated sugar, 1/2 cup light brown sugar, 4 large eggs, 2 teaspoons pure vanilla extract, 1/2 teaspoon salt, 3/4 cup all-purpose flour, 1/4 cup natural unsweetened cocoa powder
Pour the brownie batter into the prepared baking pan and bake for 32-36 minutes, just until the brownies start to pull away from the edges of the pan and a toothpick inserted into the center of the brownies comes out with only a few moist crumbs clinging to it.
Remove from oven and cool completely in the pan before transferring them to a baking sheet using the parchment paper sling for easy removal.