Preheat the oven to fan assisted 160C / 180C / 350F / gas 4 and lightly oil two 6 cup doughnut tin.
Mix together the flour, sugar, cocoa, baking powder and salt in a large bowl and set aside.
In another bowl, combine the buttermilk, eggs, peanut butter and nutella before adding in the melted butter.
Mix the wet ingredients into the dry ingredients and stir until everything is just combined.
Using a spoon, add the batter in to 8 doughnut cups until they are three quarters full.
Bake the doughnuts for 13-15 mins until am inserted skewer comes out clean, remove them from the oven and let them cool in the pan before transferring them to wire rack to cool completely.
Dip the cooled doughnuts into the melted Nutella, turn them over and cover then in sprinkles.