01 - In a small saucepan, combine the milk, diced apple, cocoa powder, brown sugar, cinnamon, and salt.
02 - Heat over medium heat, stirring frequently, until the mixture is steaming and the apple is tender, approximately 8 minutes. Avoid boiling.
03 - Remove from heat. Blend the mixture until smooth and frothy using an immersion blender or a regular blender. Return to the saucepan and keep warm over low heat if necessary.
04 - Whisk the heavy cream, vanilla extract, and powdered sugar in a small bowl until soft peaks form. Alternatively, froth using a milk frother if using oat milk.
05 - Pour the cinnamon-apple cocoa into two mugs. Top generously with the prepared vanilla foam. Garnish with a sprinkle of cinnamon and an apple slice, if desired. Serve immediately.