Preheat your oven to 350°F and generously spray or rub butter around a round cake pan or springform with nonstick spray.
In a large bowl, combine the chocolate cake mix, 1 egg, and 1 stick of melted butter.
Using an electric hand-held mixer, beat the ingredients together until fully combined.
Press the mixture evenly into the prepared baking pan, creating a smooth and even base. It will have a dry crumble texture; this is the crust of the cake.
In another large bowl, beat the softened cream cheese until smooth and creamy. If you need to soften, add it to the microwave for a few seconds.
Add the eggs, peanut butter, vanilla extract, and 1 stick melted butter, blending until smooth. Finally, mix powdered sugar with an electric mixer until well incorporated.
Spread the peanut butter topping evenly over the cake base using a rubber spatula.
Bake for about 1 hour to 1 hour and 10 minutes or until the center is slightly gooey. Remember, the center will never be entirely "done," and it's not supposed to be. This is a GOOEY cake; the center will not be completely set.
If the top of the cake is getting too brown but still needs more baking time, cover it loosely with aluminum foil. This will prevent further browning while allowing the cake to finish baking. Keep an eye on it and remove the foil during the last few minutes if needed!
Once out of the oven, let the cake sit for a bit to allow the center to settle. Cut into slices and serve on its own, or top with whipped cream or a drizzle of chocolate sauce for an extra treat!