Preheat your oven to 350 degrees Fahrenheit. Spray a 9 by 13 inch baking dish with cooking spray or line with parchment paper. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
2 1/2 cups all-purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon salt
With your mixer on medium, mix together the butter and sugar until light and fluffy, about 3 to 6 minutes.
1 1/2 cups sugar, 14 tablespoons unsalted butter
Mix in the eggs (one at a time) and then the vanilla until well combined.
2 teaspoons vanilla extract, 2 large eggs
Add the flour mixture to the wet ingredients. Mix on low until combined enough that it won't fly everywhere. Then turn up the mixer to high just long enough that you don't see streaks of flour. Scrape down the bowl and on the bottom to ensure that all the flour is added. Only mix long enough to combine the flour. (See more detailed instructions in the post.)
Press the batter into the prepared baking dish. The dough will be thick and sticky. This may make it difficult to press it into the baking dish. You can spray your hands with cooking spray or wet them with water to make this easier.
Bake for 25 to 27 minutes or until the edges are golden brown. Remove from the oven and allow to cool for 1 hour.