Preheat the oven to 350℉/180℃. Line a 9x9 baking pan with parchment paper and set aside.
In a bowl, add the flour, cocoa, baking soda, baking powder, and salt. Stir to combine and set aside.
1 1/4 cups all-purpose flour, 1/2 cup cocoa powder, 1/2 tsp salt, 1/4 tsp baking powder
In another bowl, add the butter (can be warm), oil, brown sugar, and white sugar. Whisk well to combine (2-3 minutes).
3/4 cups unsalted butter, 2 tbsp oil, 1/2 cup dark brown sugar, 1/2 cup white sugar
Add in eggs, yolk, and vanilla. Continue whisking.
2 large eggs*, 1 large egg yolk, 2 tsps vanilla
Mix in dry ingredients until a chocolate batter forms. Do not over-mix!
Stir in the chocolate chips. Do not over-mix!
3/4 cups semi-sweet chocolate chips
Transfer into prepared pan and spread evenly. Sprinkle extra chocolate chips on top. Bake for 25-30 minutes or until a toothpick inserted comes out fudgy, not with raw batter (it'll be fudgy and stick to the toothpick).
Allow to cool, slice, and enjoy!