Preheat oven to 350℉ and grease 9x9 baking dish. Set aside.
In a large mixing bowl or stand mixer fitted with the whisk attachment on low speed, combine vegetable oil, sugar, vanilla and eggs until just incorporated.
1 cup vegetable oil, 2 cups white granulated sugar, 1 teaspoon vanilla extract, 4 large eggs
Using a spatula or mixer fitted with the paddle attachment, gradually fold in flour, cocoa, baking powder and salt until well combined. Pour brownie mix into prepared baking dish using a spatula to evenly spread.
1 cup all purpose flour, ⅔ cup unsweetened cocoa powder, ½ teaspoon baking powder, ½ teaspoon salt
In a large mixing bowl or stand mixer fitted with the whisk attachment add butter, sugar, brown sugar, vanilla extract, and egg. On low speed, whisk to combine.
½ cup unsalted butter, ¼ cup white granulated sugar, 2 tablespoons white granulated sugar, ½ cup light brown sugar, 1 teaspoon vanilla extract, 1 large egg
Using a spatula or mixer fitted with the paddle attachment on low speed, gradually add flour, baking soda, baking powder, and salt, until just combined. Using a spatula, fold in chocolate chips.
1 teaspoon baking soda, ¼ teaspoon baking powder, ⅛ teaspoon salt, 1½ cup all purpose flour, ½ cup semi-sweet chocolate chips
Using a large spoon, spoon cookie dough on top spacing to see brownie peaking through.
Bake in the center rack of preheated oven 25 minutes or until a toothpick inserted into the center comes out clean.
Notes
Watch bake time carefully:
If cookie dough becomes golden in color, but brownie isn't set, gently tent with aluminum foil to prevent over-browning.
When a toothpick inserted into the center comes out clean, remove from oven and allow to cool on a baking rack.
Use your favorite 9" pie dish, cake pan, tart dish or jelly roll pan, adjusting bake time. Use the toothpick test to prevent over (or under) baking.To Store, cover with plastic wrap or place in an airtight container room temperature up to 5 days or in the freezer up to 3 months.
Substitutions
Butter - Substitute with salted butter and eliminate the added salt in the recipe.Semi-Sweet Chocolate Chips - Substitutee with dark chocolate, milk chocolate or white chocolate chips.