Step 1
In a small saucepan, cover the sweet potato with water. Bring to the boil. Cook for 15 mins or until tender. Drain and transfer to the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed for 2 mins or until pureed. (Alternatively, transfer sweet potato to a food processor and process until pureed.) Set aside to cool.
Step 2
In a medium heavy saucepan, bring butter, 1 tsp sea salt flakes and ½ cup (125ml) water to the boil over medium heat. Add flour and mix with a wooden spoon for 1 min or until a dough forms. Transfer to the clean bowl of the stand mixer and mix on medium speed for 3 mins or until cool.
Step 3
Add eggs and egg yolk, 1 at a time, beating well after each addition. Beat in ¾ cup of the sweet potato puree until just blended. Transfer dough to a piping bag fitted with a 12mm star nozzle.
Step 4
Meanwhile, to make the chocolate sauce, in a small saucepan, whisk cream, sugar, cocoa powder and a pinch of salt over medium-high heat for 4 mins or until simmering. Remove from heat. Add chocolate and vanilla. Stir until smooth. Keep warm over low heat.
Step 5
Pour enough oil into a large deep saucepan to come halfway up the side. Heat oil to 175°C. Working in batches, and holding the bag at an angle so the tip is close to the surface of oil, pipe 12cm lengths of dough into the oil, using a paring knife or kitchen scissors to cut off dough and carefully release into the oil. Cook for 2-3 mins each side or until golden all over. Transfer churros to a baking tray lined with paper towel.
Step 6
In a large bowl, mix the caster sugar and cinnamon. Toss warm churros in cinnamon sugar and serve with warm chocolate sauce.
Make it ahead
Make the chocolate sauce up to 3 days ahead, cover and refrigerate. Reheat over low heat before serving.