Preheat the oven to 350 degrees and prepare a 9X13 baking pan with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, and salt.
In an electric mixer fitted with a paddle attachment, combine melted butter, oil and sugar together on a low speed. Mix until combined.
Add eggs and vanilla. Mix until the everything is mixed together and the batter is a lighter color.
Add dry ingredients and mix on low speed until ingredients are just combined.
Fold in chopped chocolate.
Pour batter into prepared pan.
Bake 35-40 minutes or until the center no longer jiggles.
Remove from oven and place on a wire rack to cool.