Preheat the oven to 350 degrees F. Grease an 8-by-8-inch baking dish and line the bottom with parchment.
In a large bowl, whisk together the sugar, flour, salt, baking powder, and baking soda. In a medium bowl or large measuring cup, whisk together the egg, vanilla, buttermilk, oil, water, and peanut butter. Add the wet ingredients to the dry ingredients and stir to combine. Pour into the baking dish.
Bake until a toothpick inserted into the center comes out clean. Begin checking for doneness at 30 minutes.
Cool in the pan for 10 minutes. Remove to a rack to cool completely, or serve while it's still warm. Serve plain or with desired toppings.