A son's favorite agua fresca, Oatchata (oat horchata), made with rolled oats, cinnamon, evaporated milk, and vanilla, perfect for celebrating Children's Day.
Heat 3 cups water in a large saucepan over medium heat for about 5 minutes. Do not boil. Remove from heat. Add oats and cinnamon stick. Set aside for 30 minutes.
Place 1 cup water in blender. Add softened cinnamon stick from saucepan and blend on high until smooth. Run cinnamon water through a strainer into a pitcher. Set aside.
Place soaked oats with their water in blender. Blend on high until smooth. Add blended oats to pitcher with cinnamon water.
Add remaining 3 cups water, evaporated milk, sugar, and vanilla to pitcher. Stir until sugar dissolves. Serve over ice.
