How to Make It
Preheat oven to 375°F. Coat a 10 1/2- x 15 1/2-inch rimmed baking sheet with cooking spray.
Place brown sugar and butter in a medium saucepan over medium, and cook, stirring often, until butter melts and mixture is combined, about 6 minutes. Remove from heat, and let cool 10 minutes.
Whisk together eggs and brown sugar mixture in a medium bowl. Whisk in flour and baking powder until combined.
Pour batter into prepared baking sheet, and bake in preheated oven until edges are browned and start to pull away from sides of pan, 20 to 22 minutes. Remove from oven, and cool completely, about 45 minutes.