Preheat oven to 350 degrees F. and line two to four baking sheets with parchment paper.
In a large bowl, combine cake mix, sour cream, eggs, oil, and water and beat with an electric mixer until smooth. Stir in chocolate chips.
Using a generously heaping tablespoon (or you can scoop with a quarter cup measure, but not fill it completely) drop dough onto baking sheets. Bake two sheets at a time, switching racks halfway through, for about 15 minutes or until cakes appear done (they should spring back like cupcakes).
Let cool for about 5 minutes, then carefully transfer to a wire rack to cool completely.
Prepare frosting. With an electric mixer, beat the butter until soft. Add the sugar and stir until it is moistened by butter. Stir in the cocoa, sour cream and 2 tablespoons of the cream. Mix on low, then increase speed and beat until creamy. Beat in remaining cream and vanilla. Add milk a few teaspoons at a time until you reach the consistency you like. Spread over cookies.
Now this is the hard step. Leave the cookies alone for two hours. Just let them sit on the counter. You can skip this part if necessary, but I really thought they tasted better after they'd sat around for a bit.