Preheat oven to 350°F/175°C. Lightly grease a 9x13 baking dish.
Stir together the melted butter, sugars, applesauce, flax egg, and vanilla.
In a separate bowl stir together the flour, baking powder, and salt.
Stir together the flour mixture and the butter mixture. Stir until combined (with no big clumps of flour).
Spread the batter into the prepared baking dish. Bake 25 to 30 minutes. For more gooey blondies, remove them from the oven at 25 minutes. Otherwise, cook for 30 minutes.