Preheat oven to 350 degrees Fahrenheit.
Line the bottom and sides of 9x13 baking tray with two pieces of parchment and spray with cooking spray. The side pieces will serve as handles to lift the bars out of the tray when cooling.
Whisk the flour, baking powder, salt in a bowl and set aside
In the bowl of your stand mixer, using the paddle attachment, whip the butter, sugar, and salt until light and fluffy
Add the egg to the mixture and beat until combined. Scrape the sides of the bowl and mix again for a few seconds.
Add the flour mixture and mix until just combined.
Using a spatula, dollop the batter onto the bottom of the parchment lined pan. Spread into an even layer using a spatula. Set pan aside