Pour 4 cups of milk into a big pot. Then add the sugar, panela and cinnamon sticks. Stir all ingredients with a wooden spoon and bring milk to a boil over medium low heat.
When the milk comes to a boil, remove from the stove and let it sit for about 2 minutes.
Meanwhile, mix the cornstarch with the remaining milk until dissolved.
Put the pot back on the stove over medium low heat. Remove the cinnamon sticks and pour the dissolved cornstarch into the hot milk and butter. Stir constantly with a wooden spoon until it thickens and you can see the bottom of the pot.
Pour immediately into a serving dish and let it cool for at least an hour. Sprinkle with ground cinnamon.
Notes
*You can't stop stirring natilla with a wooden spoon while making it, it will burn very easy.*You can make it one day ahead and store in a glass container or a platter. Cover with plastic.