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