Preheat your oven to 350°F and grease a 9-inch round cake pan with nonstick cooking spray.
Sift the flour and cocoa powder into a large bowl. Whisk in the sugar, baking soda, baking powder and salt until well combined. Add the buttermilk, oil, egg and vanilla and whisk just to combine. Stir in the hot coffee and pour the batter into the prepared cake pan.
Heat the peanut butter in the microwave or over a double boiler until thin and liquid, drizzle over the cake batter and swirl in with a butter knife.
Bake the cake for 30 to 35 minutes or until springy to the touch and a skewer inserted into the centre comes out clean.
Allow the cake to cool in the pan for 20 minutes then turn out onto a wire rack to cool completely.