Put the cocoa powder and the sugar together in a small saucepan, and then whisk in the water. Heat the mixture on medium heat, stirring, until it is smooth and comes to a simmer.
Stir in the dissolved cornstarch and continue cooking until the syrup thickens a bit. Take off the heat and add the vanilla. Note: The cornstarch thickens the sauce and makes it more 'dippable', but you can leave it out if you like, for a thinner sauce. Let the syrup cool before using. You can store it for quite a while in the fridge.
For the fruit skewers, wash the fruit and peel, if necessary. Slice the bananas and kiwi. Cut larger fruit like melon and pineapple into chunks, and leave berries whole. Thread the fruit onto bamboo skewers. If you are serving to small children, snip off the pointed ends after you have assembled them.
If you would like to freeze the fruit, assemble the skewers and then place them on a baking sheet. Set the whole pan into the freezer and freeze until firm, this will take several hours. Remove the skewers shortly before serving. Note: For freezing use berries, grapes, banana and kiwi.