Preheat the oven to 350 degrees. Line a 9x13 pan with parchment paper. Set aside.
Melt the butter in the microwave in 20 second intervals. Once melted, set aside.
In a small bowl, melt the chocolate in the microwave in 20 second intervals, stirring after each interval. Once melted, set aside.
In a medium bowl, whisk the flour, cocoa powder, baking soda, and salt. Set aside.
In a large bowl, whisk together the granulated sugar, the light brown sugar, and the melted butter. One at a time add in each egg, whisking the egg in until just combined. (I recommend whisking the eggs in a separate bowl & pouring those eggs into the wet ingredients in 4 intervals (1 per egg). This is up to you :) )
Add the oil and the vanilla extract & whisk together.
Pour the melted chocolate into the wet ingredients & stir until combined.
In 3 intervals, add the flour mixture to the wet ingredients using a RUBBER SPATULA (this is imperative!). Only mix until all of the flour is incorporated being careful to not overmix. The batter will be thick so make sure you get all of the flour that can settle at the bottom of the bowl.
Finally, add in the chocolate chips. Again, stirring until just combined.
Spread the batter into the prepared parchment-lined pan. Bake in the oven for about 50 minutes (time depends on your oven so start checking around 47-48 minutes). They're done when a toothpick inserted in the center comes out almost clean with just a few moist crumbs.
Let it cool completely on the counter then cut & serve. Enjoy!