Preheat oven to 350 degrees Fahrenheit and prepare two 9-inch springform pans by lining the bottoms with parchment paper and greasing the bottom and sides of the pan well with butter.
Combine vegetable oil and sugar in a large bowl and stir until combined.
Whisk in eggs, one at a time, mixing until incorporated after each addition.
Add vanilla and salt and mix just until combined.
Stir in cocoa powder.
Fold in flour just until combine. Then fold in chocolate chips.
Divide batter evenly between the two prepared springform pans.
Bake at 350 degrees for 18-22 minutes (mine took exactly 20 minutes, and a toothpick inserted into the center should come out "dirty" for fudgy brownies)
Let layers cool in pan for 15 minutes, then remove from springform pan and transfer to a wire rack to let cool completely.
Once the brownie layers are cool, add 1/2 cup semi-sweet chocolate chips* and 1/4 cup heavy cream to a small saucepan.
Stir frequently over medium heat until chocolate has melted completely and the cream is incorporated.
Remove from heat and whisk in 2 tablespoons of peanut butter and 1/2 teaspoon of vanilla. Whisk until smooth.
Let cool several minutes until it is close to room temperature, but still very spreadable.
Line the sides of a springform pan with parchment paper. You may put a cake board into the bottom if you'd like as well, for easier transferring.
Remove the ice cream from the freezer to soften for just a few minutes.
Place the bottom brownie layer in the base of the springform pan.
Top with one half of your first batch of chocolate peanut butter ganache and spread evenly with an offset spatula.
Scoop ice cream on top and spread into an even layer using an offset spatula. Work quickly here! If at any point the ice cream is getting too soft, put the cake into the freezer for a while.
Top ice cream with remaining half of the first batch of ganache and spread evenly.
Place second brownie layer on top (upside down), cover with saran wrap, and place in the freezer. Freeze until firm, preferably at least 5 hours. Overnight is great!
Once cake is set, repeat steps 1-3 under chocolate peanut butter ganache to make the second half-batch of ganache. Let cool until the ganache drips fluidly, but slowly off a spoon. It should be slightly warm, but not hot.
Remove cake from springform pan and dispose of parchment paper.
Pour ganache over the top of the cake and spread, using an offset spatula, until it just barely drips over the sides.
Return cake to freezer for about 15 minutes until ganache sets.
Top with chopped peanut butter cups. Return to freezer until ready to serve.
*Because of the necessity to freeze the ice cream cake before decorating, plan to make the ganache in two half batches (half to go between the layers when stacking, and half to pour on top at the end after freezing.)