Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large mixing bowl, combine the melted butter and brown sugar.
Add eggs one at a time, mixing well after each addition.
Stir in the vanilla extract until fully incorporated.
In a separate bowl, whisk together flour, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
If using, fold in the white chocolate chips.
Pour the batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for 25-30 minutes or until golden brown.
Allow to cool in the pan before cutting into squares and dusting with powdered sugar.