1. In a heavy 2-quart saucepan, mix cocoa powder and sugar.
2. Over medium-high heat, gradually whisk milk and 1/2 cup of the whipping cream until smooth. Bring to a boil, reduce heat to low and simmer gently 25 to 35 minutes until cocoa is reduced to slightly less than one cup; stir often and adjust heat as needed, being careful not to boil.
3. Meanwhile, whip remaining 1/2 cup whipping cream and the sour cream to form soft peaks; refrigerate.
4. Remove cocoa mixture from heat and immediately add the chopped chocolate; stir until chocolate is completely melted and the mixture is smooth.
5. Spoon 2 tablespoons hot cocoa mixture into each of the four 8-ounce, straight-sided glasses.
6. Top each with a scoop of ice cream and 2 tablespoons of whipped cream; repeat layer once, ending with whipped cream.
7. Serve immediately with spoons.