Position a rack in the center of the oven and preheat to 350°F (180°C).
Grease a 9-by-13-inch (23-by-33-cm) dark metal pan with softened butter, then line with parchment paper, leaving overhang on all sides. Grease the parchment with more butter.
OPTION 1: If using prepared mix, make the brownie batter according to the package instructions.
OPTION 2: If making the brownie batter from scratch, in a large bowl, whisk together the sugar, cocoa powder, salt, baking powder, and flour. Add the vegetable oil, vanilla, eggs, and water and whisk to combine.
Stir in the walnuts.
Pour the brownie batter into the prepared baking pan and smooth the top with a spatula.
Pour the peanut butter over the top and swirl into the batter with a knife or skewer.
Bake until lightly puffed on top, about 20 minutes. Transfer the brownie pan to a wire rack and let cool completely, about 30 minutes.
Use the parchment paper to lift the cooled brownies out of the pan and cut into 12 bars.
Serve with a dollop of whipped cream and a pinch of sprinkles.