Preheat the oven to 350ºF.
Grease two 9" cake pans with butter. Set aside.
In a stand mixer fitted with the paddle attachment, combine the cake mix, instant pudding mix, eggs, sour cream, water, vegetable oil and espresso powder mixed with water. Beat on low speed, scraping down the sides, until the ingredients are throughly combined.
Stir in the chocolate chips then divide the batter evenly between the two prepared pans.
Bake the cakes on the center rack for 20-25 minutes, or until a toothpick inserted comes out clean.
Allow the cakes to cool in the pans for 20 minutes, then invert them onto a cooling rack and allow them to continue to cool while you make the frosting.