To Make The Peanut Butter Flourless Chocolate Cake:

1. Adjust an oven rack to the lower-middle position and heat the oven to 325°F. Line the bottom of an 8-inch springform pan with parchment paper or waxed paper and grease the sides of the pan. (Be sure to grease the sides really well!)
2. Place a large roasting or baking pan on the bottom rack of the oven.  Bring a kettle or medium-sized pot of water to boil on the stove.  Once it comes to a boil, carefully pour the water into the baking pan in the oven.  This will be your water bath, and will help prevent the top of the cake from cracking.
3. While the water is coming to a boil, add the eggs to the bowl of a stand mixer.  Using the whisk attachment, beat the eggs at high speed until the volume doubles. This usually takes about 5 minutes.
4. Meanwhile, melt the chocolate and butter and peanut butter together. You can either do this in a double boiler on the stove (by placing the chocolate and butter and peanut butter in a large heatproof bowl, set over a pan of almost-simmering water, and stirring until melted and smooth). Or you can do this in the microwave (by heating the chocolate and butter and peanut butter in a microwave-safe bowl in 30-second intervals, stirring in between, until the ingredients are melted and smooth). Then fold about a third of the beaten eggs into the chocolate mixture using a large rubber spatula until only a few streaks of egg are visible. Fold in half of the remaining egg foam, and then the last half of the foam, until the mixture is totally homogenous.
5. Scrape the batter into the prepared springform pan and smooth the surface with a rubber spatula. Place the springform pan on the middle rack of the oven (a rack above the pan of boiling water). Bake until the cake has risen slightly, the edges are just beginning to set, a thin-glazed crust (like a brownie) has formed on the surface, and an instant-read thermometer inserted halfway into the center reads 140° F, about 23-26 minutes.
6. Turn the oven off and crack the door oven open a few inches, and wait about 10 minutes for the cake to slightly cool.  Then remove cake and transfer to a wire rack, and cool until it reaches room temperature.  Then cover and refrigerate until the cake is completely chilled. (The cake can be refrigerated for up to 4 days.)
7. Once the cake has chilled, carefully remove the sides of the springform pan, invert the cake onto a sheet of waxed paper, peel off the parchment paper, and then re-invert the cake onto a serving platter. Top with the peanut butter chocolate ganache swirl, if desired.  Or you can serve the cake lightly dusted with powdered sugar, or just plain.

To Make The Peanut Butter Chocolate Ganache Swirl:

1. Melt the chocolate and heavy cream together in a double-boiler or in the microwave (use the same directions as above), and whisk together until the mixture is completely smooth.
2. Add the peanut butter to a separate bowl, and heat in the microwave (about 15-25 seconds) until it is warmed and a little melty.  Transfer the peanut butter to a pastry bag or a large ziplock bag, and then use scissors to snip off a small corner of the bag so that you can pipe the peanut butter.  Set aside.
3. Pour the chocolate mixture on top of the chilled cake, and use a spoon to spread it out in an even layer.  Pipe the melted peanut butter on top of the chocolate in some long lines (or your desired pattern).  Then immediately take a wooden skewer (or a toothpick or a knife) and drag it through the peanut butter in circles to create swirls.  If you’d like, you can then run a knife around the edge of the cake to smooth out the sides, or you can just leave it drippy.
4. Refrigerate the cake for about an hour to let the ganache firm up, or you can just serve it immediately.