If you're making this cake with your child prepare a small cooking pan, and all the ingredients.
Let your child place butter, sugar, cocoa powder, and water in the pan.
Now it's your turn. Put the pan on your cooker, and melt the ingredients, stirring constantly, until mixed into chocolate sauce. Don't boil them! Leave them to cool slightly. Take 1/4 of the cup of chocolate sauce, an leave it on the side, to pour it over the cake when it's finished.
While you are doing this, your child can prepare the flour and baking powder, by sieving them together into one bowl.
Pour your cooled down chocolate sauce from the pan into a mixing bowl. Let your child add to it: flour, eggs and vanilla extract. Mix it well.
Pour your cake mixture into prepared cake tin, and bake it in 150°C for 50 mins.