In a large pot or casserole, mix the condensed milk, water, and milk. Heat gently over medium heat.
While the milk mixture heats, rub the butter into the cake flour with your fingers until it resembles coarse crumbs.
Once the milk is hot (not boiling), slowly stir in the flour mixture.
Stir continuously until the mixture thickens and the flour is fully cooked (±10 minutes).
Serve warm with cinnamon sugar sprinkled on top.