Preheat oven to 350 degrees. Line a 9 x 13 pan with nonstick cooking spray and parchment paper and set aside.
In a large pot over medium-low heat, melt butter, oil, chocolate, and cocoa together until fully melted and smooth. Remove from heat.
Pour in sugar and vanilla extract. Add in eggs, 1 at a time, stirring vigorously before the next addition. After all four eggs have been mixed in, the batter should be thick and smooth without that granular look to it. Add in the remaining dry ingredients and stir gently until just combined. Do not over mix.
Fold in milk chocolate chips and walnuts if using, and pour batter into prepared pan. Smooth the top and bake 30-35 minutes. Cool at least 15 minutes before cutting and serving. Or, cool completely and frost with this recipe.