Beat the cake mix, instant pudding mix, eggs, sour cream, butter, vanilla extract, and 1 1/4 cup milk in the bowl of a stand mixer for 2 minutes. Pour into a lightly greased 4-quart slow-cooker. Meanwhile, heat 2 cups milk in a saucepan for 3 to 5 minutes. Do not boil. Sprinkle the dry Cook and Serve pudding mix over the cake mixture in the slow-cooker. Slowly pour milk over the dry pudding mix. Cover and cook on LOW for 3 1/2 hours. During the cooking time, toast the pecans in a non-stick skillet for 3 to 5 minutes. After 3 1/2 hours, turn off the slow-cooker and spinkle with pecans, marshmallows, and chocolate chips. Let stand for 15 minutes before serving.