Stir 1/4 cup of the skim milk, the cornstarch and salt together in a 2 quart saucepan until completely dissolved and smooth. Add remaining milk, coffee, instant coffee granules, and sweetened condensed milk. Cook over medium-high heat whisking constantly. The mixture will begin to gain volume and thicken slightly. When this happens, remove it from the heat source.
In a small bowl, beat the egg and gently temper two tablespoons of the hot mixture into the egg. Return the saucepan to the heat source and gradually stir in the tempered egg mixture, whisking constantly. The addition of the egg will make the mixture lose much of it's foaminess. Continue to heat the mixture until a few bubbles burble to the top. Cook for 2 minutes longer after this happens, again whisking constantly.
Pour into 5 dessert glasses; cover and refrigerate. Top with whipped cream if desired. Sprinkle with cinnamon just before serving.
Keyword hot coffee, instant espresso powder, sweetened condensed milk