Prepare your cake mix in two eight-inch, round cake pans, according to the package directions. Set them aside to cool.
In a glass mixing bowl, beat the peanut butter, powdered sugar and salt until it is fluffy and well combined.
Pop the bowl into the microwave for a few seconds (start with 20) to thin the frosting out a bit before frosting your cake.
Place one of your cakes onto a plate and slather a thin layer of frosting on top. Top with the other cake and cover the whole layer cake to your liking with the rest of the frosting. If it's still difficult to work with, microwave it just a bit more. Just be careful not to make it so runny that you have the opposite problem to thick frosting.
Serve your cake immediately or store in an airtight container.