To make the churro dough: Combine 1 cup of water with the butter or margarine and the salt in a saucepan and bring to a boil over high heat. Using a wooden spoon, stir in flour. Reduce the heat to low and stir vigorously until the mixture forms a ball, about 1 minute. Remove the dough from the heat and, while stirring constantly, gradually beat the eggs into the dough.
To make the chocolate for dunking: In a small bowl, dissolve the cornstarch in 1 cup of milk and reserve. Combine the chocolate with the remaining cup of milk in a saucepan. Stirring constantly, melt the chocolate over medium-low heat. Whisk the sugar and the dissolved cornstarch into the melted chocolate mixture. Reduce the heat to low and cook, whisking constantly, until the chocolate is thickened, about 5 minutes. (Add extra cornstarch if it doesn't start to thicken after 5 minutes.) Remove the pan from the heat and whisk until smooth then reserve in a warm place.
Heat about 2 inches of oil in a heavy, high-sided pot over medium-high heat until the oil reaches 360 degrees F. Mix the sugar with the cinnamon on a plate and reserve.
Meanwhile, spoon the churro dough into a pastry bag fitted with a large tip. Squeeze a 4- inch strip of dough into the hot oil. Repeat, frying 3 or 4 strips at a time. Fry the churros, turning them once, until golden brown, about 2 minutes per side. Transfer the cooked churros to a plate lined with paper towels to drain.
When the churros are just cool enough to handle, roll them in the cinnamon-sugar (in Spain churros are simply rolled in sugar.)
Pour the chocolate into individual bowls or cups. Serve the warm churros with the chocolate dip.
Professional Recipe: This recipe was provided by a chef, restaurant or culinary professional and makes a large quantity. The Food Network Kitchens chefs have not tested this recipe in the proportions indicated and therefore cannot make any representation as to the results.
Recipe courtesy of Chocolateria San Gines, Madrid Spain