1. Preheat the oven to 325 degrees F (300 degrees F if you are using a dark or non-stick pan). Butter a 9" springform pan and set aside.
2. To make the crust, stir the Oreo crumbs, melted butter and salt in a small bowl with a fork until the crumbs are moistened. Press the crumbs evenly over the bottom and up the sides of the springform pan. (I like to use the bottom a measuring cup to press the crumbs around the pan. I've found that they get evenly distributed that way and end up looking neater around the sides.) Freeze the crust for 10 minutes.
3. Bake the crust for 10 minutes, then transfer it to a rack and let cool.
4. To make the fudge, put the chopped chocolate in a bowl. Heat the cream to a simmer over medium heat. Immediately pour the simmering cream over the chopped chocolate. Let stand for 1-2 minutes, then add the vanilla extract and stir with a whisk or rubber spatula until smooth and combined. Pour the fudge into the cooled crust and spread evenly over the bottom. Place in freezer for 10 minutes. Remove from freezer and sprinkle surface with the chopped peanut butter cups. Return to freezer for 10 more minutes.
5. Meanwhile, prepare the cheesecake filling. Beat the cream cheese and peanut butter together on medium speed until smooth. Add the sugar and vanilla extract and continue to beat on medium speed until thoroughly combined. Reduce the speed to low and add the eggs one at a time, beating until combined after each addition. Scrape the bowl and mix on low for another 20 seconds.
6. Remove the pan from the freezer and pour the cheesecake filling over the fudge and peanut butter cups. Bake until the sides of the cheesecake are set and the middle just slightly jiggles, about 1 hour. Transfer to a wire cooling rack and cool to room temperature (at least 1 hour). Refrigerate the cheesecake at least 4 hours or overnight, until thoroughly chilled. To unmold the cheesecake, wrap a warm towel around the outside of the springform pan to loosen the crust from the sides. Carefully remove the sides from the springform pan.
7. Before serving, prepare the ganache topping. Place the chopped chocolate in a small bowl. Bring the heavy cream to a simmer over medium heat. Immediately pour the simmering cream over the chopped chocolate. Let sit for 1-2 minutes and then stir with a whisk or rubber spatula until smooth. Pour the ganache on the top of the cheese and, using an offset spatula, spread evenly over the top of the cheesecake. Return to the refrigerator to set the ganache, about 20 minutes. Garnish as desired (I had some leftover peanut butter icing from filling a customer cake, so I piped that around the border).