Preheat oven to 350°F and spray an 8x8 or 9x13 pan with cooking spray or coat with butter.
In a large bowl, mix 1 cup butter(melted) and 2 cups dark brown sugar.
Add 2 large eggs and 2 teaspoon vanilla extract and stir vigorously until smooth.
Add 1 ¾ cup all-purpose flour, ½ teaspoon baking powder, and ½ teaspoon 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.