There's just no substitute for old-fashioned, homemade chocolate pudding that you can whip up before dinner and let cool until you're ready to serve.
In a medium saucepan, whisk together cornstarch, sugar, cocoa, cinnamon, and salt. Combine 1 1/4 cups cream with milk. Whisk 1 cup cream mixture into dry ingredients until cornstarch is dissolved. Whisk in remaining cream mixture; place saucepan over medium-high heat.
Cook, whisking constantly, until mixture comes to a boil and thickens, about 5 minutes. Add chocolate; cook, whisking, 1 minute more. Remove from heat, and whisk in butter until melted. Transfer to a bowl and place a piece of plastic wrap directly on pudding to prevent a skin from forming. Let stand until lukewarm, about 45 minutes.
Whip the remaining cup of cream to soft peaks. Divide pudding among four bowls, and serve with whipped cream. Lightly sprinkle cinnamon on whipped cream, if desired.
This pudding can also be refrigerated and served cold with heavy cream poured on top.