Rinse the apple well. Using a sharp knife and sturdy chopping board, carefully cut the apple in half. 

Use a spoon or melon baller to remove the core and seeds from one half. Chop the cored apple half into small chunks, about 2 cm in size.

Add the frozen chocolate milk cubes, plain milk, peanut butter, chopped apple pieces, cinnamon powder, and cocoa powder (if using) to the blender jar.

Ensure the jar is securely attached to the base.

Start the blender at a low speed and gradually increase to a high. 

Blend for 1-2 minutes, until the mixture is completely smooth and the apple is fully pureed, with no chunks remaining. 

If needed, stop the blender midway and use a spatula to scrape down the sides of the jar, then resume blending.

Once the shake is smooth and creamy, pour it into a tall glass. Serve immediately, while it's still chilled and frothy. 

If desired, garnish with a sprinkle of cinnamon or a drizzle of peanut butter.