Pre-heat oven to 325 degrees Fahrenheit. Line a 9x13 baking dish with parchment paper and spray with non-stick spray. Then, melt butter in medium saucepan over medium heat.
Keeping the heat on medium heat, the butter will slightly bubble. Let the butter keeping bubbling, stirring often. Once the butter stops foaming and browned bits form at bottom of pan (about 10-15 minutes total). Remove from heat.
Immediately stir in 12 ounces of semi-sweet chocolate chips or chunks. Stir until chocolate is fully melted and incorporated with the butter.
In a stand mixer with paddle attachment, add eggs, sugar, salt and vanilla and mix until combined.
Then, add in vegetable oil and continue to mix until fully combined. Next, add in chocolate butter mixture and mix until just combined.
Add in flour to the chocolate mixture. Mix until just combined. A few streaks of flour are okay!
Remove bowl from stand, add in chocolate chips/chunks (and/or nuts). Stir with silicone spatula or wooden spoon until chocolate chips or chunks are evenly distributed.
Turn batter into prepared baking pan. Smooth the batter with an offset spatula or silicone spatula.
Bake for 15 minutes, turn the baking dish, then bake for another 15-20 minutes or until top is completely set.
Remove from oven and cool to room temperature. Cut into 12-15 pieces (or smaller) and enjoy.