In a mixing bowl, combine the flour, tapioca flour, baking soda and salt.
In a large saucepan, combine the water and brown sugar and bring to a boil, then add the margarine and heat until melted. Lower the heat to low, stir in the vanilla, and pour in a little of the flour and stir; it will be very thick , but just keep stirring. After all the flour has been added, remove from the heat and continue to stir for about a minute, to form a thick, smooth dough. Let the dough cool for 10 minutes.
While the dough is cooling down, heat oil for deep frying on medium high heat. Make sure that there is at least 2" of oil in the pan, to cook the churros properly, they should be able to float in the oil.
Fit a large pastry bag with a large star tip nozzle. Scoop the batter into the bag; firmly twist the top so that the dough drops down the bag.
Spread the sugar on a plate, stir in the cinnamon and keep ready.
Check if the oil is hot enough by dropping ¼tsp of dough and if it starts to bubble rapidly and fries quickly, then you are ready to make some churros.
To make the churros, twist down the top of your pastry bag, squeeze the top and press the length of dough about 4" long directly into the hot oil. Use a sharp scissors or your hands to pinch off the churros from the pastry bag.
Depending on the size of your pan, you can fry 5~6 churros at a time, make sure not to crowd the pan.
Use either metal tongs or a metal slotted spoon to gently turn the churros occasionally.
Fry for 4~6 minutes or until the churros are firm and lightly golden. Remove from the oil onto a paper towel lined plate to drain.
Cool for 2~3 minutes, then place them in the cinnamon sugar mixture and gently roll to coat. Serve warm with hot chocolate.