Add all ingredients to a medium sized pot, and turn heat to medium/low. Stirring the ingredients consistently just until the dough begins to pull away from the sides, only 3-5 minutes. Turn out onto a clean surface and let cool slightly, before kneading the dough. Let cool down before handing to children to play with. Can store in an airtight container. 

Add the water in a medium sized pot, and turn heat to medium high to bring water to a light boil. Once boiling, reduce heat to medium low and add the baking soda. Be careful as this will cause the hot water and soda to bubble up quite a bit. Give it a good stir, and then add your cornstarch and cocoa powder. Stirring consistently for a few minutes just until the dough begins to form and pull away from the sides of the pan. Turn out onto a clean surface and let cool slightly, before kneading the dough. Let cool down before handing to children to play with. Can store in an airtight container.