Preheat oven to 350 degrees F.
Line a 9-by-13-inch square baking pan with nonstick foil.
In a small bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
In a microwavable bowl, melt butter and chopped chocolate.
Cool 5 mins.
Stir in sugar.
Add eggs, yolk, and vanilla until smooth.
Fold in dry ingredients.
Add chocolate chips and spread into pan.
Bake for 25 to 30 minutes.
Cool.