In a bowl cream together 1/2 stick of the butter, the peanut butter, and the brown sugar. Beat in 1 of the eggs and the cream cheese, a little at a time, beating until the mixture is smooth.
In a small saucepan melt 1 stick of the remaing butter with the chocolate over low heat, stirring until the mixture is smooth, and let the chocolate mixture cool.
In another bowl cream together the remaining 1 stick of butter and the granulated sugar, beating until the mixture is light and fluffy, and beat in the remaining 2 eggs, 1 at a time, beating well after each addition.
Stir in the chocolate, the vanilla, and the flour, sifted, and pour the batter into a buttered 13-by 9-inch baking pan.
Drop dollops of the peanut butter mixture into the batter, swirling to marble the batter. Bake the brownies in the middle of a preheated 350 degree oven for 45 to 50 minutes, or until they pull away slightly from the sides of the pan and tester comes out with crumbs adhering to it. Let brownies cool before cutting into squares.