Preheat oven to 425 degrees.
Spray two 6-ounce ramekins with nonstick cooking spray and dust with cocoa powder. This ensures the cakes will easily come out of the ramekins when inverted onto a plate after baking.
Chop the chocolate (or you can use equivalent amount of chocolate chips). Place butter into a medium heat-proof bowl, then add chopped chocolate on top. Microwave on high in 10 second increments, stirring after each until completely smooth. Set aside.
Whisk the flour, powdered sugar, espresso powder, and salt together in a small bowl. Whisk the eggs, egg yolks and vanilla extract together until combined in another small bowl. Pour the flour mixture and egg mixture into the bowl of chocolate. Slowly stir everything together using a rubber spatula or wooden spoon. The batter will be slightly thick.
Spoon chocolate batter evenly into each prepared ramekin. Top with peanut butter and use a spoon to slightly press it down into the batter. Some may still be exposed
Place ramekins onto a baking sheet and bake for 12-13 minutes until the sides appear solid and firm. The top may still look slightly soft.
Allow to cool for 1 minute, then cover each with an inverted plate and turn over. They will be hot, so be careful. The cakes should release easily from the ramekin.
Top with ice cream and melted peanut butter for serving, if desired. Serve immediately.Cakes can also be rewarmed for a few seconds in the microwave, if you want to prepare in advance. Just enough to make the peanut butter filling slightly oozy.