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.
6 ounces cream cheese, 2/3 cup creamy peanut butter, 1/3 cup granulated sugar, 2 large eggs, 1/2 teaspoon pure vanilla extract
To make the brownie layer, 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.
3/4 cup unsalted butter, 3 ounces unsweetened baking chocolate, 1 cup light brown sugar, 1/2 cup granulated sugar
Add eggs, one at a time, stirring well after each addition. Add vanilla, stir well. Add flour and salt and stir until there are no traces of flour left.
3 large eggs, 1 teaspoon pure vanilla extract, 3/4 cup unbleached all purpose flour, 1/4 teaspoon salt
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 onto the top of the peanut butter mixture.
Insert an 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 wire rack until completely cooled.
Using an actual bar of unsweetened chocolate instead of cocoa powder gives you the richest flavor in brownies, which is exactly what we want. You can substitute with a bar of bittersweet baking chocolate if desired.
Creamy peanut butter such as Jif or Skippy is best for this recipe. Natural peanut butters tend to separate and aren't typically stable enough which in turn, can affect the texture of the finished brownies.
We used a 10-inch skillet, which makes the brownies nice and thick. You can use a larger skillet if that's all you have on hand, but do note they will likely take less time to bake through and the brownies themselves will not be as thick. Bake until a toothpick inserted into the center comes out clean.