Begin by preparing the ganache. Place the chocolate and peanut butter in a small bowl. Heat the cream in a small saucepan over medium-low heat until it's barely simmering. Pour the hot cream over the chocolate-peanut butter mixture, and let it sit for a few minutes. Stir to combine, and refrigerate while you prepare the remaining ingredients.
Preheat the oven to 425* F. Butter your skillet(s) or ramekins, and sprinkle with either sugar or cocoa powder to prevent sticking. Set aside.
Place the butter and remaining chocolate in a small saucepan. Heat, stirring constantly, until the chocolate is completely melted. Alternatively, you can do this in the microwave. Let it cool for a few minutes.
Meanwhile, beat the sugar, eggs, and vanilla in a medium mixing bowl until the mixture is pale yellow and tripled in volume, about five minutes. Gently fold in the melted chocolate until completely mixed. Fold in the flour and salt.
Divide half of the batter between the skillet(s) or ramekins. Then, divide the ganache into the center of each skillet or ramekin, and top with the remaining batter.
Bake for about 12-15 minutes, or until the top is crackly but the center is still slightly jiggly. You can now either serve it straight out of the skillet(s), or you can invert the cakes onto plates. Don't forget the ice cream!