Makes 1 quart (1 L)
Put the milk, sugar, cinnamon, and baking soda into a saucepan. Bring to a boil, then lower the heat and simmer for about 20 minutes, or until the sugar has melted completely and the mixture is reduced to about 3 cups (750 ml). Set aside to cool a little.
Meanwhile, beat the egg yolks until they form thick ribbons on the beater-about 10 minutes, depending on the efficiency of your beater.
Remove the cinnamon from the milk mixture. Gradually add the milk mixture to the egg yolks, still beating. Return to the pan and cook over low heat, stirring and scraping the bottom and sides of the pan constantly, until the mixture thickens enough to coat the back of a wooden spoon. (Take care, as it can quickly turn to scrambled eggs. At the first sign of this tragedy, pour into the blender and blend until it is smooth.) Set aside to cool completely.
Meanwhile, sterilize a 1-quart (1 l) bottle in boiling water.
Gradually stir the alcohol into the “custard,” pour into the bottle, and use immediately or store in the refrigerator up to one month for future use.