Preheat oven to 350°F. Line an 8x8 or 9x9 pan with foil and spray with cooking spray.
Stir oil, sugar, cocoa, eggs, vanilla, and water until smooth. Stir in flour, then stir in chocolate chips. Spread in prepared pan and bake for 23-26 minutes until they just start to loose their glossy sheen. (I like mine a little under done.) Cool completely before continuing.
Stir together sweetened condensed milk, peanut butter, vanilla, and whipped topping. Stir in peanut butter cups.
Use foil to carefully remove the entire sheet of brownies from the pan. Line the same pan with plastic wrap, making sure that the plastic overhangs the edges and that the plastic comes up the sides of the pan all the way.
Remove the foil from the brownies and place the brownie slab carefully back in the pan, on top of the plastic. Spread ice cream mixture on top. Wrap up the plastic wrap, adding more as needed, to completely cover the ice cream layer. Freeze for at least 4 hours or overnight before serving. Store in freezer.