This peanut butter ice cream cake takes the cake of all ice cream cakes (get it?). With three layers of thick fudge, chocolate ice cream, peanut butter cup ice cream, and a crunchy pretzel crust, you won't be able to resist this sweet and salty combination.
In a food processor, blend the pretzels until a fine crumb forms. Pulse in the melted butter until combined. Press the pretzel crumbs into the bottom of a springform pan, pressing slightly up the sides. Place in the freezer for 10-15 minutes to set up.
Spread 1 cup of fudge on top of the pretzel crust. Freeze for an additional 15 minutes (this freezing helps the ice cream spread easier).
Remove chocolate ice cream from the freezer about 10 minutes before adding it to the cake. Spread the chocolate ice cream on top of the first fudge layer. Freeze for 30 minutes to 1 hour.
Spread 1 cup of fudge on top of the chocolate ice cream. If the ice cream didn't freeze through enough, it may start melting as you add the fudge and come through during the spreading. That's ok, just work around it or continue spreading (but this is why the additional freeze is recommended after each layer). Place in the freezer for 15 minutes while preparing the next layer.
Remove the vanilla ice cream from the freezer for 10 minutes to soften slightly. Stir in the chopped peanut butter cups until thoroughly combined. Spread on top of the fudge layer and return to the freezer for "the big freeze". Freeze for at least 4 hours, or overnight (preferred).
Before serving, spread the final 1 cup of fudge on top of the cake. Top with additional peanut butter cups and/or pretzels as desired. Freeze again for 30 minutes if you want the fudge to be more firm, or serve immediately. Drizzle with salted caramel if desired.
Notes
If the pretzel pieces are too big, the crust will have trouble sticking together. You don't want them processed into dust, but a fine crumb. The good news: even if the crust does fall apart a little, it still tastes so dang delicious.