Preheat your oven to 350°F (175°C). Grease your baking pan or line it with parchment paper for easy removal. In a mixing bowl, combine the vegetable oil and granulated sugar, whisking until smooth.
Mix in the eggs one at a time, followed by the vanilla extract and water. Continue to whisk until everything is well combined.
Sift together the cocoa powder and all-purpose flour, then fold them into the wet ingredients. Be careful not to overmix; a few lumps are okay!
Gently fold in the chocolate chips, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, spreading it evenly. Bake for about 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let it cool in the pan.
While the brownies are cooling, in a separate bowl, combine the sweetened condensed milk, peanut butter, and vanilla extract. Mix until smooth and well incorporated.
Gently fold in the whipped topping or fresh whipped cream until the mixture is light and fluffy.
Once the brownies are completely cool, spread the peanut butter ice cream mixture over the brownies. Top with chopped peanut butter cups for a delightful crunch.
Cover the pan and freeze for at least 4 hours, or until the ice cream layer is firm. When ready to serve, slice into squares and enjoy!