Preheat oven to 350 degrees F. Spray a 12x15-inch pan with nonstick spray.
Prepare the bars: In a large bowl, mix together the bar ingredients. Stir until well combined. Pat this layer evenly into the pan.
Bake for 20 minutes (should be still soft). Remove from oven and cool slightly before covering with frosting.
Prepare the frosting: Combine the brown sugar, cream and butter in a saucepan and bring to a hard boil for 1 minute. Add the vanilla and stir in enough powdered sugar until the mixture is spreadable (approx. 1 pound). Spread on top of the cooled bars. Let the frosting set before cutting (you may with to refrigerate the bars until slightly firm for neater cutting).
You may double the frosting recipe if you desire more frosting.