Preheat the oven to 325 F. Lightly grease the bottom and sides of a 10" cast iron skillet.
In a mixer at medium speed, cream together cream cheese, peanut butter, and sugar. Beat for 2 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Cover and set aside.
Cut butter into 6 chunks. Place butter and chopped chocolate into a bowl and melt in the microwave. Add both sugars to melted chocolate and stir until everything is combined. Add eggs, one at a time, stir well after each addition. Add vanilla, stir well. Add flour and salt and stir until there are no traces of flour left.
Reserve one cup of the brownie batter and set aside. Pour remaining brownie batter into prepared skillet.
Spread the peanut butter mixture over the brownie batter.
Spoon the remaining one cup of brownie batter in dollops on to the top of the peanut butter mixture.
Insert icing spatula or butter knife into the batter (spatula should be straight up and down) and swirl the mixture. Do not overdo it, start from one side and finish at the other, no need to go over it again.
Bake in preheated oven for 55-65 minutes (mine was 63). The center will puff up a bit and the edges will pull away from the sides of the pan. Allow to cool in pan on a cooling rack until completely cooled.