Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking pan with butter or cooking spray.
In a large bowl, whisk together the melted butter and sugar until smooth.
Add the eggs one at a time, whisking well after each addition.
Stir in the vanilla extract until fully combined.
Sift together the flour, cocoa powder, and salt in a separate bowl.
Gradually fold the dry ingredients into the wet mixture until just combined.
Fold in the chocolate chips (if using).
Pour the batter into the prepared baking pan and smooth the top.
Bake for 25-30 minutes, or until a toothpick inserted comes out with moist crumbs.
Allow to cool in the pan before cutting into squares.
