In a large bowl, combine melted butter, brown sugar, egg, and vanilla. Stir until evenly combined. Sprinkle flour, baking powder, baking soda, and salt over the mixture, and stir until just combined. The batter will be very sugary and thick.
1/3 cup unsalted butter, 1 cup packed brown sugar, 1 large egg, 1 and 1/2 teaspoons vanilla extract, 1 cup plus 2 tablespoons all-purpose flour, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon table salt