Horchata de Avena (Guatemala)

This is one of my favorite drinks from Latin America. It is very refreshing but also more filling than just plain old juice.

1 cup ground oatmeal (hard to find in the states, but you can substitute regular oatmeal)
4 cups water
1/2 cup milk
1 cup sugar
1/2 tsp vanilla

Soak the oatmeal in 1 cup water, add sugar. Leave it for one hour, pass it through a colander. Add three more cups water, milk, and vanilla. Serve chilled.
Option: You can also add some cinnamon.

Recipe by Clemencia de Chaluleu