1. In a large bowl, whisk together instant coffee powder, castor sugar, cocoa powder, whip cream powder, and milk powder until thoroughly combined.
2. Transfer the mixture into an airtight jar and store in a dry place for up to 20 days.
3. When ready to prepare the iced mocha, place ice cubes in a blender jar. Add 3 tablespoons of the prepared mixture and 1 cup of water. Blend until smooth.
4. Pour the blended mixture into a serving glass.
5. Garnish with grated chocolate or a sprinkle of coffee powder on top.
6. Serve immediately and enjoy the refreshing taste of homemade iced mocha.