In a 2-quart saucepan, whisk together the sugar, cocoa, cornstarch, and salt.
Slowly whisk in the 2 cups oat milk until smooth and combined. Stir the mixture constantly over medium-low heat. Continue stirring until it begins to bubble and thicken. The pudding should coat the back of the spoon (about 10-15 minutes).
Add the chopped bittersweet chocolate and stir it into the pudding until fully combined, and the pudding is thick and smooth (about 1 minute).
Remove from heat and stir in the vanilla and instant espresso until the espresso is fully dissolved. Spoon into a bowl, cover, and chill for about 3 hours before eating. (You can eat warm too if preferred, but it will thicken as it cools.) Top with cool whip and a dash of cinnamon, and enjoy!