Preheat oven to 350 degrees and spray a 9x9 pan with cooking spray or coat with butter.
In a large bowl, mix melted butter and brown sugar. Add in the eggs and vanilla and whisk vigorously until smooth.
Add the flour, baking powder, and salt and stir until well incorporated.
Pour the batter into the prepared pan and smooth out.
Place in the oven to bake for 30 minutes until edges are golden and toothpick comes out mostly clean.
Allow to cool completely, then cut into squares.