To make the cookie layer, preheat the oven to 350 degrees F.
Spray a 12 inch cast iron skillet with cooking spray.
In the bowl of an electric mixer fitted with the paddle attachment, cream together butter, peanut butter, and sugars until fluffy.
Add in the egg and vanilla and mix until combined. Scrape down the sides of the bowl if you need to.
In another medium bowl, mix together the flour, baking soda, and salt. Add this to the peanut butter mixture and mix until completely blended.
Press the cookie dough into the prepared skillet and bake for 15-20 minutes, until just barely done. Remember that cast iron retains heat well and the cookie will continue to cook for a few minutes after you remove from the oven, so you don't want to overbake it.
Cool the cookie for 15 minutes.
For the chocolate layer, mix the chocolate chips and sweetened condensed milk in a small saucepan over medium heat. Stir until completely melted and blended.
Pour the chocolate over the cookie and spread evenly.
Refrigerate for an hour.
To make the marshmallow frosting, whisk together the egg whites, sugar, and baking soda in the top pan of a double boiler set over simmering water on medium heat. Whisk until the sugar is dissolved and the mixture is warm to the touch, about five minutes.
Pour the egg mixture into the bowl of an electric mixer fitted with the whisk attachment. Start mixing on a low speed, then work up to medium high, and mix until you reach stiff peaks, about 6 minutes.
Add in the vanilla and stir.
Spread the frosting over the chocolate layer. Burn the frosting with a kitchen torch and serve.