Preheat your oven to 350°F (180°C) and line an 8x8 baking pan with parchment paper.
In a medium mixing bowl, whisk together the flour, baking powder, and salt. Set aside. 3 ¼ cups (390 g) all purpose flour ¼ teaspoon fine sea salt ¾ teaspoon baking powder
In the bowl of a stand mixer fitted with the paddle attachment cream on medium speed butter, granulated sugar, and brown sugar, until light and fluffy, about 2 to 3 minutes. 1 cup (2 sticks, 226 g) unsalted butter ½ cup (107 g) packed light brown sugar ½ cup (100 g) granulated sugar
Add the egg and vanilla extract and mix until combined, scraping down the bowl as needed. 1 large egg 2 teaspoons vanilla extract
Add the dry ingredients into the stand mixer and mix on low speed until combined. The dough should start to pull away from the sides. Be careful not to overmix. Stop mixing once the last of the flour has been incorporated.
Reserve 2 cups of dough (loosely filled do not press down into the cup).
Add the remaining dough to the 8x8 baking pan pressing down into an even layer making sure to spread the dough all the way to the sides.
Bake the dough for 15-20 minutes or until the center is set and the edges are very light brown. Do not overbake. Once the crust is done, set aside to cool.