With the rack in the middle position, preheat the oven to 325°F (165°C). Butter an 8-inch (20 cm) square baking pan and line the bottom with a sheet of parchment paper, letting the paper hang over two sides.
In a bowl, combine the flour, cocoa powder and baking powder. Set aside.
In a large bowl placed over a pot of simmering water, melt together the dark chocolate, butter, peanut butter and 1/2 cup (125 ml) of the condensed milk. Stir until just melted and smooth.
Remove the bowl from the pot, then whisk in the brown sugar and eggs. Stir in the dry ingredients. Pour half of the mixture into the prepared pan.
Peanut Butter Filling
In another bowl placed over the simmering water, melt the white chocolate. Add the peanut butter and the remaining condensed milk. Stir until just smooth. Remove the bowl from the pot. Spread the filling over the brownie mixture already in the pan, then cover with the remaining brownie mixture.
Bake for 40 minutes or until a toothpick inserted into the centre of the brownies comes out with a few crumbs attached. Let cool completely on a wire rack. Unmould.
Cut into squares. The peanut butter brownies will keep for 1 week in an airtight container at room temperature. Serve drizzled with caramel sauce and topped with salted peanuts, as desired.