Position a rack in the middle of the oven and preheat to 350 degrees. Grease a 9-inch-square baking pan with nonstick cooking spray, then line the bottom with a piece of parchment paper large enough to have a generous overhang on two sides.

Combine the butter, 1 cup of the granulated sugar and the brown sugar in the bowl of a stand mixer or hand mixer; beat on medium speed for a few minutes, until lightened and a little fluffy. Stop to scrape down the bowl.

Add the eggs and vanilla extract; beat on medium speed until well incorporated.

Whisk together the flour, cinnamon, baking powder and salt in a separate bowl, then add to the mixer bowl; beat on medium-low speed until well incorporated. The dough will be soft.

Transfer to the baking pan, then spread the dough evenly all the way into the corners. Whisk together the remaining 2 tablespoons of granulated sugar with the amount of cinnamon you prefer; we recommend ½ teaspoon. Sprinkle that mixture evenly over the surface. Bake for about 25 minutes, or until golden and still slightly soft at the center.

Cool completely (in the pan) before serving or storing. It's best to lift out the slab and slice on a cutting board; discard the parchment before cutting.