Preheat the oven to 180°C.
Sift the flour, sugar, cocoa, and bicarbonate of soda into a large mixing bowl.
Melt the butter and cola in a pan and stir into the dry ingredients with the milk, eggs, and vanilla.
Pour the mixture into a buttered 24cm loose-based cake tin and bake for 40 minutes.
Melt butter, cola, and cocoa in a pan, bring to a boil, then beat into the icing sugar.
Pour the smooth topping over the warm cake and allow it to cool in the tin.
Melt the chocolate bar with cream and water to create a pouring sauce.
Stir in marshmallows if using and serve with the cake.