If you feel your hot cocoa came out too thin, use milk with a higher level of fat. If you used skim milk, try reduced fat or whole milk instead. Or use half oat milk and half milk. You can also use sweetened condensed milk or heavy whipping cream. However, this will significantly increase the calories, fat, and sugar. For reference, 1 cup of heavy cream has over 800 calories compared to oat milk and regular dairy milk which have about 100 calories per cup.