Preheat the oven to 180°C (355°F). Line a 9" (23cm) square baking pan with parchment paper overhanging.
Add the butter, both sugars, yogurt and vanilla to a large bowl. Mix until combined and some of the sugar has dissolved.
Add the all-purpose flour, cornstarch, raising agents, and salt and mix until just combined. The dough should be very soft and tacky. Add all the chocolate and mix through.
Scoop the cookie dough into your prepared pan and smooth the top. If desired, top with additional chocolate chunks/chips.
Bake the cookie bars for 25-28 minutes or until they're evenly golden brown on top. For softer cookie bars, bake for less time. Place the baking pan on a wire rack and let the cookie bars in the pan cool for 45 minutes to 1 hour.
When the cookie bars are room temperature, remove them from the pan, cut into bars and enjoy!
Store leftover cookie bars in an airtight container at room temperature for 3-5 days or in the refrigerator for up to 1 week. The bars also freeze well for up to 1 month.