For the brownies: Line a 9-by-13-inch baking pan with foil, pressing it into the corners and up the sides; leave an overhang on 2 sides. Butter the foil and dust with flour. Position a rack in the lower third of the oven; preheat the oven to 350 degrees F.
Melt the butter in a large saucepan over medium-low heat. Add the bittersweet chocolate and cook, stirring constantly, until mostly melted. Remove from the heat and continue to stir until the chocolate is completely melted.
Add the granulated sugar, cocoa powder and salt to the saucepan and stir until combined. Add the eggs 1 at a time, stirring well after each addition, then stir in the vanilla. Add the flour and stir until incorporated.
Pour the brownie batter into the prepared pan and spread it to fill the pan. Bake until a toothpick inserted in the center comes out with just a few crumbs, about 40 minutes.
Let cool in the pan on a rack, about 30 minutes. Lift the brownies out of the pan using the foil and transfer to a cutting board to cool completely before frosting.
For the peanut butter frosting: Using an electric mixer on medium speed, cream together the confectioners' sugar and butter until light and fluffy. Then beat in the peanut butter and cream cheese.
Frost the top of the brownies. Add 1 teaspoon water to the jam in a small bowl and stir to combine. Dollop the jam evenly on top of the frosting and use a butter knife to gently swirl it in. Cut into squares and serve.