Combine the chocolate cake mix, sour cream, vanilla, butter,eggs and 1 ¼ cups of milk in a large bowl (or directly in the crock pot);Transfer to slow cooker.
In a separate bowl, combine the pudding and remaining 2 cups of milk and whisk as directed on the package. – Pour directly on top of the prepared cake mix. Do not stir.
Secure the crock pot lid and cook. Remove the lid and sprinkle on the mini marshmallows and chocolate chips. Put the lid back on and allow to cook for 15 minutes.