Heat the oven to 350 degrees.
In the mixing bowl of a stand mixer, place the brownie mix, 1/3 cup of oil, 1 cup of water, and 2 eggs, and blend on low until all ingredients are well blended.
Turn mixer on high, and mix for 1 minute.
Scrape down the sides of the mixing bowl ad blend any unmixed ingredients.
Line the bottom and sides of a 9 X 13 baking dish with parchment paper.
Pour the Brownie batter into the dish, and set aside.
Rinse out the mixing bowl, and add the cream cheese, butter, peanut butter and vanilla, and mix until well blended, smooth and creamy.
Add the sweetened condensed milk, egg, and cornstarch, and blend until well mixed.
Remove the bowl from the mixer, and add the chopped peanut butter cups, gently folding them into the cream cheese mixture.
Pour the cream cheese mixture on top of the brownie batter, and place in the oven for 45 - 50 minutes, or until a toothpick inserted in the center comes out clean.