Preheat the oven to 350°F, and spray a 9 x 9-inch or 8 x 8-inch baking pan with cooking spray. Set aside.

Place the butter in a medium saucepan, and melt over low heat. Add the sugar, vanilla, and eggs (in that order to keep the eggs from coming in direct contact with the warm butter). Whisk to combine.

Sift the flour, cocoa powder, and salt over the pan, and whisk until just combined. Don’t overmix; otherwise, the brownies will be dense and tough. 

Stir in the baker's chocolate, and stir until melted. Transfer the batter to the prepared baking dish. Bake for 35 to 45 minutes or until a toothpick inserted in the center comes out clean or with a couple of crumbs.

Remove from the oven, and allow to cool before serving.

Store leftovers in an airtight container for up to 4 days.