Preheat oven to 350 degrees.

Lightly grease an 8x12-inch baking pan* with cooking oil spray. Line with parchment paper (or baking paper); set aside.

In a bowl, whisk the melted butter, oil and sugars together. Add in the eggs and vanilla.  Whisk in until lighter in color.

In another bowl, sift the flour, cocoa powder and salt. Gently fold the dry ingredients into the wet ingredients until JUST combined (do NOT over combine).

Fold in 3/4 of the chocolate pieces. Pour batter into prepared pan, smoothing the top out evenly, and then sprinkle with remaining chocolate pieces.

Bake at 350 degrees for 25-35 minutes for just under-done brownies (until the center of the brownies no longer jiggle).

Allow to cool in the pan for 15-20 minutes. Then carefully remove them out of the pan and allow to cool to room temperature before slicing into 16 brownies.