Makes about 12
1 cup plain flour
½ tsp orange rind, finely grated
1 tbsp caster sugar
½ cup caster sugar, extra
2 tsp ground cinnamon
vegetable oil, for deep-frying
250g dulce de leche
Combine butter and 1 cup of water in a saucepan over medium heat. Cook, stirring, for 2 minutes or until butter melts and mixture just comes to the boil. Add all the flour at once, stirring until combined and mixture comes away from the side of the pan. Remove from the heat and transfer to a bowl.
Use an electric mixer to beat the flour mixture while adding eggs, one at a time, beating well between each. Add orange rind and 1 tbsp sugar, and stir to combine.
Combine extra sugar and cinnamon in a shallow bowl.
Spoon flour mixture into a piping bag fitted with a 2- centimetre fluted nozzle.
Heat enough oil in a deep frying pan to reach a depth of 5 centimetres. Heat to 180°C. Pipe two 10-centimetre lengths of batter into hot oil. Cook for 1 minute each side or until golden brown and cooked through. Use a slotted spoon to transfer to a plate lined with paper towels. Immediately transfer to cinnamon sugar, toss to combine.
Split doughnuts lengthways. Spoon dulce de leche in the centre of each to serve.