Preheat your oven to 350 degrees Fahrenheit and position the rack in the center. Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides for easy removal. Grease the pan before adding parchment to help it stick in place.

Melt the butter in a medium saucepan over low heat, or microwave in 30 second bursts until fully melted but not bubbling or browned. Let it cool for about 5 minutes before proceeding.

Pour the slightly cooled melted butter into a large mixing bowl. Add the sugar and whisk vigorously for about 2 minutes until the mixture looks slightly lighter.

Crack in the eggs one at a time, whisking well after each addition. Add the vanilla extract with the last egg. The mixture should look smooth and glossy.

Sift the Hershey's cocoa powder directly into the bowl to prevent lumps. Fold it in gently using a spatula or wooden spoon until no dry streaks remain.

Whisk together the flour, salt, and baking powder in a small bowl. Add this to the chocolate mixture and fold until just combined. Stop mixing the moment you see no more white streaks.

Fold in chocolate chips or your chosen mix ins if using. Pour the thick batter into your prepared pan and spread it evenly with a spatula. Bang the pan gently on the counter a few times to release air bubbles and level the surface.

Bake for 28 to 32 minutes until a toothpick inserted in the center comes out with a few moist crumbs attached. The edges should look set while the center appears slightly underdone.

Remove the pan to a wire rack and let the brownies cool completely, about 1 to 2 hours. Use the parchment overhang to lift them out, then slice with a sharp knife.