Preheat oven to 325 degrees. Spray a 9-inch round cake pan with cooking spray. Line with parchment paper and spray parchment and sides of pan with cooking spray.
In a medium saucepan, cook butter, chocolate, and peanut butter over medium low heat, stirring constantly. You could also do this in a microwave safe bowl, cooking in 30 second intervals stirring in between.
Remove from heat and whisk sugar into butter for about 2 minutes. Add the egg yolks, one at a time, whisking well in between. Add a pinch of salt.
In a medium bowl, beat the egg whites on high with a mixer until stiff peaks form. Gently fold egg whites into the chocolate mixture until combined. Pour into prepared pan.
Bake for 45-60 minutes. You want the middle a little on the underdone side. Remove from the oven and cool on a rack for 1 hour. Refrigerate for 4 hours. You can serve cold or warm. I like to heat mine up in the microwave for 30 seconds and serve with ice cream or whipped cream.
Notes
If you're not a huge peanut butter fan you can definitely leave it out and it will not affect the cake.