1. Preheat the oven to 400°F (200°C) and grease 2 baked doughnut pans (or use one in batches).
2. Whisk the sugar, buttermilk, oil, eggs and vanilla together in a large mixing bowl.
3. In a separate bowl, sift the flour, cocoa powder, baking powder and salt. Ad this all at once to the liquid and whisk until well-combined (the batter will be fluid).
4. Transfer the batter to a pitcher and slowly pour the batter into each doughnut cup, filling ¾ full. Bake for 8 to 10 minutes until the doughnuts spring back to the touch, but there is no browning. Cool the doughnuts for 10 minutes in the pan and then tip out onto a rack to cool completely.
5. For the frosting, beat the cream cheese and butter together (using electric beaters, a stand mixer with the paddle attachment or by hand) until smooth and fluffy and then beat in the peanut butter. Add the icing sugar and vanilla and beat until smooth. Transfer this to a piping bag fitted with a star tip and pipe the frosting onto the cooled doughnuts. Top with sprinkles and enjoy
The doughnuts are best enjoyed the day they are baked, but will keep, refrigertated and covered for up to 2 days, but let the doughnuts warm up on the counter for 30 minutes before serving