Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4 and grease two donut tins with the butter
Mix together the caster sugar, milk, vegetable oil, Nutella and eggs in a bowl with a whisk
Add the self raising flour and cocoa powder, and whisk in
Stir in the chopped roasted hazelnuts if using
Divide the batter between the trays. You can put the batter into a piping bag to make it a bit neater to dispense into the trays, but you can do it with a spoon too
Bake them for 12 minutes, then remove them from the tin (I use a spoon to gently prise them out) and leave to cool on a cooling rack
Set the donuts onto a rack with a baking tray or some baking paper underneath to catch any drips
Warm the Nutella for 30 seconds in the microwave, stir then warm for longer if needed
Dip the top half of the donuts in the melted Nutella and place back on the rack
Sprinkle on the chopped roasted hazelnuts if using, then leave to set - or dig straight in!
Store leftover in an airtight container and eat within 3 days