Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a mixing bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth.
Add the eggs and vanilla extract, and whisk until well combined.
In a separate bowl, mix the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips, if using.
Spread the batter evenly in the prepared baking pan.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool in the pan before cutting into squares.