Preheat your oven to 325°F. Prepare an 8x8-inch baking pan by lightly spraying it with baking spray and lining it with foil or parchment paper.

In a microwave-safe bowl, combine the butter and chopped semi-sweet chocolate bar. Heat in 30-second increments, stirring after each, until the mixture is completely melted.

Stir in both sugars until well blended. Next, whisk in the eggs and vanilla extract until the mixture is glossy.

Add the cocoa powder, flour, salt, and chocolate chips. Fold the dry ingredients into the wet mixture until just combined. The batter will be thick, but that's a good sign!

Spread the batter evenly into your prepared pan. Bake for 35-45 minutes. Around the 35-minute mark, start checking for doneness by inserting a toothpick into the center—if it comes out with wet batter, it needs more time. If there are just a few moist crumbs clinging to it, it’s ready. Check-in at 2-minute intervals if needed.

Once baked, cool the brownies in the pan on a wire rack for about 2 hours. Use the foil or parchment as handles to list the brownies from the pan.

Slice into nine squares and serve.