Preheat oven to 350°F (175°C). Grease an 8x8 inch baking pan.
In a bowl, mix melted butter and sugar until combined.
Add eggs and vanilla extract, mix well.
In another bowl, whisk together flour, cocoa powder, baking powder, and salt.
Gradually add dry ingredients to wet ingredients, mix until just combined.
Fold in chocolate chips if using.
Pour batter into prepared pan and spread evenly.
Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean.
Let cool before cutting into squares.