In a small saucepan over medium heat, whisk together water, sugar, salt and butter. Bring the mixture to a boil then remove it from the heat. Stir in the flour and ¼ cup ground cereal, mixing until it forms a ball.
Heat vegetable oil in a large, heavy-bottomed deep pot set over medium-high heat. There should be about 3 inches of oil in the pan as well as 3 inches above the oil to prevent it from bubbling over. Heat oil until it reaches 375 degrees F.
Transfer the dough to a cloth or heavy-duty pastry bag fitted with a large star tip. Pipe about 6 inches of dough over the pot of oil and fry until golden-brown. Depending on the size of your pot, you can cook two or three churros at a time.
Transfer the churros to a paper towel-lined plate. Cool for about 2 minutes, then roll in the cinnamon-sugar mixture. Repeat process with remaining dough.