Lightly spray a 1 1/2-quart souffle dish with butter-flavored cooking spray.
Make a foil ring, 1-inch thick, for souffle dish to sit on inside a 5-quart slow cooker.
In a medium bowl, combine brownie mix, 2 eggs, vegetable oil, and 1/4 cup creme de menthe. Stir using 50 strokes or use a hand blender on medium speed for 1 minute to combine. Fold in chopped mint thins. Transfer to prepared souffle dish and set aside.
In a medium bowl, combine cream cheese, remaining egg, sugar, and 2 teaspoons creme de menthe. Using an electric mixer beat on low speed until smooth. Spread over brownie mixture. Use a butter knife to cut cream cheese mixture into brownie mixture for a marble effect.
Place souffle dish on top of foil ring in slow cooker. Lay 8 paper towels over top of slow cooker bowl and secure with lid. (This helps to trap steam.)
Cook on LOW setting for 4 1/2 to 6 hours. (Do not lift lid to check brownies for the first 3 hours.)
Allow brownies to cool completely before cutting.