Heat water, sugar, butter, and salt in saucepan and bring to a boil. Once it starts to boil, remove from heat. Stir in flour until ball forms.
Let cool for a few minutes then place in stand-up mixer with paddle attachment or large mixing bowl. With the mixer on medium speed, pour in eggs. Beat for 4 full minutes. The dough will separate at first, but eventually all come together.
Fill a large skillet with vegetable oil. Heat over medium heat for 10-15 minutes or until temperature of oil reaches 350ºF. (Check temperature with candy thermometer because if oil is too hot, churros will burn on outside, but not be cooked inside.) Mix cinnamon and sugar topping together in a bowl large enough to dip cooked churros in. Set aside.
Meanwhile, while oil is heating, place churro dough in a pastry bag fitted with large star tip. Hold the pastry bag about 6 inches above oil with knife dipped in oil in other hand. Pipe out about 4 inches and then cut off dough from pastry tip with knife. I also used the knife to shape dough back into straight line if dough curves in oil. I was able to cook 3 at a time. Fry for 3-4 minutes, flipping with tongs occasionally to cook both sides. (Hint: Especially if you are using gluten free flour, make sure to cook whole time. Even if they look done on the outside, they need time to cook on inside. I periodically checked the temperature of the oil, so I knew my cooking times were accurate.)
Use a slotted spatula or spoon to remove from oil. Transfer to a baking sheet lined with paper towels to absorb extra oil. Then use fork to toss and roll churros in cinnamon/sugar mixture. Set on another baking sheet and repeat process with remaining dough.
Fill another pastry bag with a circle tip attached with chocolate hazelnut spread or other desired filling. Use a wooden skewer to poke a hole through middle of churros. Pipe the filling into one end, flip churro around, and fill in other end until filling is throughout the middle.
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Equipment needed: tongs, slotted spoon or spatula, 2 baking sheets (one lined with paper towels), large skillet for frying, candy thermometer, and pastry bag with star and circle tipsMake Ahead: These can be made ahead. Place cooled churros in a gallon freezer bag and freeze until ready to eat (or gift). Set out at room temperature to thaw and then warm in microwave for a few seconds or lay on a baking sheet in a single layer to reheat in warm oven. Adapted from Food Network