How to Make It
Preheat the oven to 350°. Grease a 9-by-13-inch metal baking pan with baking spray. Line the bottom with parchment paper and grease it with baking spray.
In a small bowl, combine both of the sugars and the salt. Using your fingertips, rub the vanilla seeds into the sugar mixture until incorporated.
In a medium bowl, using a hand mixer, beat the butter with the sugar mixture at medium speed, scraping down the side and bottom of the bowl, until well blended, about 2 minutes. Beat in the egg and vanilla extract, then beat in the flour in 3 additions, mixing just until incorporated. Fold in the chopped chocolate. Scrape the batter into the prepared pan and, using your fingertips, press into an even layer. Bake in the center of the oven for about 30 minutes, until golden and set. Let cool. Cut into bars and serve.