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.