Preheat oven to 350 degrees. Spray a 9×13 pan with nonstick cooking spray and then line with parchment paper. Set aside.
In a large pot, over medium-low heat, melt butter, oil, chocolate, and cocoa together. Mix until smooth then remove from heat.
Pour in sugar and vanilla extract. then mix. One at a time, add in eggs. Next, gently stir in the remaining dry ingredients until just combined. Do not over mix.
Fold in milk chocolate chips and walnuts if using,. Pour the batter into prepared pan and smooth the top. Bake 30-35 minutes then cool at least 15 minutes before cutting and serving. Or, cool completely and frost.