Preheat the oven to 350 degrees F. Grease a 9- by 9-inch pan with butter.
Whisk together the sugar and eggs in a large bowl. Whisk in the butter, cocoa and vanilla. Stir in the flour and salt. Add the chopped semisweet and bittersweet chocolates, bacon and pecans and stir to combine.
Spread the batter into the pan.
Bake until the brownies are moist but not wet, testing with a toothpick, 25 to 30 minutes. They will be very fudgy. Scoop and serve warm or cool completely before cutting.