Preheat oven to 350 degrees and spray an 8x8 or 9x13 pan with cooking spray or coat with butter.
In a large bowl, mix melted butter and brown sugar.
Add eggs and vanilla and stir 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 mins (for 9x13) or 40-45 minutes (for 8x8) until edges are golden and toothpick comes out mostly clean. Don't just put in the oven and forget about it, keep checking.