Preheat oven to 350°F. Spray a 10” x 5” rectangle pan (or 9” x 9” square pan) with baking spray and line with parchment paper, leaving a couple of inches of overhang so the finished brownies can be lifted easily from the pan.

In a large microwave-safe bowl, combine butter and chocolate. Microwave in 30 second intervals, stirring between each, until butter and chocolate are melted and smooth.

Add both sugars and whisk well to combine.

Add cocoa powder, flour, salt, eggs, and vanilla. Stir until ingredients are just combined. Fold in chopped chocolate or chocolate chips.

Pour batter into the prepared pan and bake for 35 to 40 minutes, or until center is set. (If you like your brownies slightly underbaked, take them out around 35 minutes.)

Let brownies cool in pan, then cut into squares to serve. Store covered at room temperature.