Brush your ramekins with butter or spray, then dust lightly with the cocoa powder. Place on a baking sheet and set aside. Preheat your oven to 450 degrees.
Place your butter and chocolate in a microwave-safe bowl. Microwave in 30-second increments. Stirring after each one. Once the chocolate is almost all the way melted, remove and stir until it is all melted. Set aside.
Add eggs, egg yolk, and brown sugar to a bowl. Mix with a hand mixer until light and fluffy. This can also be done with a whisk if you prefer, it is a great workout. Pour in the chocolate mixture and vanilla, and mix to combine.
Divide the batter between the 4 ramekins. Drop 1-2 tablespoons of peanut butter into the center of the mixture. I used 2 tablespoons per cake, you may want less. In that case, do 1 tablespoon per cake.
Move the baking sheet to the oven. Bake for 12 minutes. You want the outside to be slightly set and the center soft.
Remove from the oven, and allow the cakes to sit for about 1 minute. Slide a knife around the edges to make sure the cakes will slide out easily. Invert carefully onto a plate and slide the ramekin off.
Dust with powdered sugar, if you choose. Then top with ice cream or whipped cream, again, if you choose.
Serve immediately, as the molten cake will continue to cook and you won't have the gooey center.