GREASE the bottom half of the slow cooker pot with butter or cooking spray. Cut a piece of wax paper to fit the bottom of the pot. Grease the wax paper as well.
MIX together the butter, eggs, sugar, brown sugar and vanilla until creamy using a stand mixer.
STIR the flour, baking soda, and salt together in a separate bowl. Turn the mixer to low speed and slowly add flour mixture. When mixture is incorporated, add chocolate chips, marshmallows and nuts until well-blended.
SPOON the dough evenly into the slow cooker pot and smooth top with spatula.
COVER and cook on LOW for 2:30 hours or until toothpick inserted into center comes out clean. Turn off the heat and remove pot from slow cooker base. Let cool in pot for 30 minutes before transferring cookie to cooking rack.
SERVE slice of cookie with a dollop of JET-PUFFED Marshmallow Creme on top.