Preheat oven to 350 degrees Fahrenheit. Grease and line a 13"x9" pan with parchment paper, then set aside.
In a large microwaveable bowl, combine the butter and chocolate. Heat at 50% power in the microwave at 30 second intervals, stirring between each, until chocolate is melted.
In another large bowl, whisk together the eggs, salt, sugars, and vanilla extract.
Pour the egg mixture into the melted chocolate/butter, stirring constantly, until well incorporated.
Stir in the flour, mixing only until just incorporated.
Spread into the prepared pan and bake on the middle rack for 45 minutes, or until there is a nice shiny crust on the top.
Cool completely before cutting.