Preheat oven to 350℉. Coat bottom of 13 x 9-inch pan with nonstick cooking spray or shortening and a flour for dusting.
In a large mixing bowl, add cake mix, coffee, oil, eggs, chocolate chips, and vanilla. Beat for 2 minutes. Pour batter in pan.
Bake for 34 to 39 minutes, when toothpick inserted in the center comes out clean. Place cake on cooling rack and cool for 15 minutes. Remove cake from pan and let cool 60 minutes, frost with Buttercream Frosting.
For Buttercream Frosting: Combine butter and powdered sugar in a large bowl mix with electric mixer. Add vanilla and milk to make desired consistency. Beat for 1 to 2 minutes until very creamy. Spread over cool cake.