Preheat the oven to 325 degrees F.  Lightly mist a 9 by 9-inch pan with non-stick spray, and line the bottom with a square cut from parchment paper.

Place the butter, cream cheese, and sugar in a large mixing bowl, and beat on medium-high speed until very pale and fluffy (3 to 5 minutes).

Add the eggs, one at a time, scraping the bottom and sides of the bowl with a silicone spatula after each addition.

Add the flour and salt, and mix on medium-low speed until just barely combined.

Stir in the vanilla extract, and transfer the batter to the prepared pan.

Bake for 30 to 40 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with a few moist crumbs.  Cool completely.

In a small mixing bowl, stir the peanut butter and powdered sugar together until smooth.

Spread the mixture in an even layer, over the top of the cooled cake.

Place the cream in a large liquid measuring cup.

Add the chocolate, and microwave in 15 second intervals, stirring, until the ganache is smooth.

Pour over the cake, smoothing with an offset spatula as needed.