Preheat oven to 375 degrees F. Grease an 8-inch round cake pan with nonstick cooking spray. Cut a piece of parchment to fit the bottom of the pan. Place it in the bottom of the pan and spray it with nonstick cooking spray. Set aside.
To make the cake, put the chocolate and butter in a large microwave-safe bowl, and heat until the butter is melted and the chips are soft, about one minute. Stir until the chocolate is melted and the mixture is smooth. If you need to reheat, do for 10 seconds at a time and stir. Add the sugar, salt, and vanilla extract and stir to combine.
Add the eggs and stir until smooth. Add the cocoa powder and stir until just combined. Don't over mix.
Pour the batter into the prepared pan and bake the cake for 25 minutes or until the cake has a thin crust on the top and the center registers 200°F on an instant-read thermometer.
Let the cake cool on a wire cooling rack for 10 minutes. Loosen the edges of the pan with a butter knife and carefully turn it upside down onto a cake plate or serving plate. The bottom of the cake will now be the top of the cake. Let the cake cool completely.
While the cake is cooling make the peanut butter frosting. With a handheld mixer or stand mixer, beat the butter on medium speed for about 1 minute. Add the peanut butter, confectioners’ sugar, heavy cream, vanilla extract, and salt. Beat on low speed until combined and then increase to high speed and beat until smooth and creamy, about 3 minutes. You may need to scrape down the sides of the bowl with a spatula. If the frosting is too thin, add a little more confectioner’s sugar. If the frosting is too thick, add a little more cream.
Next, make the chocolate ganache. Combine the chocolate and cream in a small microwave-safe bowl. Place in the microwave and heat for 30 seconds. Remove and stir. If the chocolate isn’t melted, place back in the microwave for 15 seconds. Remove and stir again, until smooth. If there are a few unmelted chocolate chips, that is fine, keep stirring until they melt into the mixture. If you need to put the ganache back in the microwave, try 10 seconds.
To assemble the cake, frost the top of the cake with the peanut butter frosting. Don’t frost the sides. Spread the chocolate ganache evenly over the peanut butter frosting. I leave a little edge so you can see the peanut butter frosting. Garnish with chopped up peanut butter cups. Let the glaze set up for a few hours before cutting and serving the cake. You can put the cake in the refrigerator to speed up the process.
Cut the cake into slices and serve.