Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking pan with parchment paper and spray with cooking spray.
Combine the granulated sugar, brown sugar, vanilla, salt and eggs in a large bowl and mix until well combined; set aside.
Melt the butter and chocolate in a double boiler, then whisk together until fully combined. Pour the chocolate mixture into the sugar mixture and mix until fully combined. Fold in the flour and cocoa.
Pour the batter into the prepared baking pan and bake until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Let cool completely before slicing.