In a medium sized mixing bowl combine half a cup of unsweetened cocoa powder and one cup of sugar, and mix until they are well blended. You may use a sifter to mix the sugar and cocoa powder, to make sure there are no lumps.
Now, take a small saucepan and pour half a cup of water into it. Add seven tablespoons of cornstarch to the water, and mix until the cornstarch is dissolved completely.
Gradually add the cocoa and sugar mixture to the saucepan, stirring continuously with a hand whisk.
Keep whisking the wet and dry ingredients together, until the mixture attains the consistency of a thick paste.
Now, set the saucepan on the stove and cook over medium high heat, slowly adding the milk. Make sure the consistency of the mixture remains constant throughout the time when you are adding the milk.
Once you have added all four cups of milk to the saucepan, allow the hot chocolate mixture to simmer for 8 to 10 minutes, stirring continuously.
After having allowed it to simmer for 10 minutes, your hot chocolate should have the consistency of loose pudding, and a nice glossy look.
Remove the hot chocolate mixture from the heat and allow it to cool and thicken for some time before serving.
Ladle the hot chocolate into small serving mugs, and serve with butter cookie sticks or plain shortbread, which can easily be dipped into the chocolate. You can also serve hot Spanish chocolate with sugar coated churros.