Preheat the oven to 350°F. Grease a 9-inch round cake pan and line the bottom with parchment paper.
Whisk together the flour, cocoa powder, sugar, baking soda, baking powder, and salt.
In another bowl, whisk the eggs, buttermilk, oil, and vanilla. Add the wet ingredients to the dry ingredients and mix until combined
Slowly whisk in the hot water. The batter will be thin.
Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Cool the cake completely before assembling the ice cream layer.
Let the peanut butter ice cream soften until it is thick and spreadable. Stir in 1 cup of chopped peanut butter cups.
Place the cooled cake in a clean cake pan or springform pan. Spread the softened ice cream evenly over the cake.
Drizzle 1/3 cup peanut butter over the ice cream layer. Sprinkle with some additional chopped peanut butter cups.
Cover tightly and freeze for at least 4 to 6 hours, preferably overnight.
To make the topping, gently heat the chocolate chips and heavy cream until smooth. Stir in the peanut butter.
Top with the remaining chopped peanut butter cups and an optional peanut butter drizzle.
Freeze for another 20 to 30 minutes, then let the cake rest at room temperature for 5 to 10 minutes before slicing.
Let the chocolate mixture cool slightly, then spread it over the frozen cake.
Use a sharp knife warmed under hot water and dried completely to cut clean slices.
