In a large mixing bowl, combine the vegetable oil, granulated sugar, and unsweetened cocoa powder. Whisk together until fully combined and smooth.
Add the eggs, 1 teaspoon of vanilla extract, and water to the mixture. Whisk again until the mixture is well-blended and glossy.
Gently fold in the all-purpose flour and chocolate chips using a spatula. Make sure not to overmix – just combine until you no longer see any dry flour.
Preheat your oven to 350°F (175°C). Pour the brownie batter into a greased 9x13 inch baking pan and spread it evenly. Bake for 20-25 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 15-20 minutes. This cooling period is essential for the layering process.
In a mixing bowl, combine the sweetened condensed milk, peanut butter, and 1 teaspoon of vanilla extract. Mix until smooth and creamy.
Once the brownies have cooled, spread the peanut butter ice cream mixture evenly over the brownies. You can use a spatula or the back of a spoon for this.
Spread the whipped topping or fresh whipped cream over the peanut butter ice cream layer, smoothing it out to cover completely.
Sprinkle the chopped peanut butter cups evenly over the whipped topping layer for an extra crunch and burst of flavor.
Refrigerate the brownies for at least 4 hours (or overnight for best results) to allow the layers to set. Once set, slice into squares and enjoy your Peanut Butter Ice Cream Brownies!