¾ cup (170 grams) unsalted butter, melted¾ cup (165 grams) firmly packed light brown sugar½ cup (100 grams) granulated sugar1 large egg (50 grams), room temperature1 large egg yolk (19 grams), room temperature1½ teaspoons (6 grams) vanilla extract2 cups (250 grams) all-purpose flour1 teaspoon (3 grams) kosher salt½ teaspoon (2.5 grams) baking soda¼ teaspoon (1.25 grams) baking powder 3⁄4 cup (129 grams) coarsely chopped bittersweet chocolateVanilla Buttercream (recipe follows)Garnish: finely chopped bittersweet chocolate
Preheat oven to 350°F (180°C). Spray a 10-inch round cake pan with baking spray with flour.
In a medium bowl, whisk together melted butter, sugars, egg, egg yolk, and vanilla until combined.
In another medium bowl, whisk together flour, salt, baking soda, and baking powder. Fold flour mixture into butter mixture until almost completely combined. Fold in coarsely chopped chocolate until combined and no dry streaks remain. Press dough into an even layer in prepared pan.
Bake until surface is dry and edges are lightly browned, 25 to 30 minutes. Let cool completely in pan on a wire rack. Remove from pan, and transfer to a serving platter.
Spoon Vanilla Buttercream into a pastry bag fitted with a small open star piping tip (Ateco #824). Pipe buttercream around edge of cooled cookie. Garnish with finely chopped chocolate, if desired. Store in an airtight container for up to 3 days.