Begin by combining one pack of white gelatin powder with 750ml of cold water in a saucepan. Mix well until the powder dissolves completely. Place the saucepan over medium heat and add the cocoa powder, whisking continuously to prevent lumps. Let the mixture come to a boil and cook for just 1-2 minutes, being careful not to let it overflow. Add the sugar, stir until dissolved, then remove from heat immediately.
Pour the hot mixture into a rectangular container, ensuring it's about 1 inch thick. Let it cool at room temperature for 30 minutes, then place in the freezer for 10 minutes to speed up the setting process. Once set, remove from the freezer and cut the jelly into ½-inch cubes (make them smaller if serving to children). Set the cubes aside.
In a large mixing bowl, combine the chilled all-purpose cream, milk, condensed milk, and vanilla flavoring. Mix these ingredients together until smooth – you can use a hand mixer for extra smoothness, but a whisk works fine too. If you want the chocolate version, add the chocolate syrup or Milo powder and mix until well combined.
Gently fold the chilled jelly cubes into the cream mixture. Transfer everything to a container and refrigerate for 2-3 hours before serving. For a more drinkable consistency, you can add extra cold milk until you reach your desired thickness. Serve in glasses, preferably chilled.
Remember to use cold water for the gelatin to prevent lumps, and make sure all your dairy ingredients are well-chilled before mixing. The finished dessert will keep in the refrigerator for up to three days.