Preheat oven to 325°F. Grease a 9x13-inch pan, line with parchment paper, and then grease the parchment paper. Alternately, use a silicone pan.
In a medium bowl, whisk together butter, olive oil, sugar, eggs, vanilla and salt until smooth.
In a small bowl, mix together cassava flour, cocoa powder, baking soda and cream of tartar. Add flour mixture to egg mixture, and stir to form a smooth batter. Fold in walnuts and dark chocolate.
Spread batter evenly over prepared pan. Bake for 15 minutes. Remove from oven and let rest for 10 minutes. Add back to the oven and cook for another 15 minutes.