Spray slow cooker with nonstick cooking spray. In a large bowl, combine brownie mix with ¼ cup melted butter, 2 eggs, ½ teaspoon vanilla and a pinch of salt. Fold in chocolate chips and set aside.
In a separate large bowl, combine chocolate chip cookie mix with remaining ¼ cup butter, 2 eggs, ½ teaspoon vanilla and a pinch of salt.
Into the slow cooker, drop alternating spoonfuls of brownie and cookie batter until all batter has been used. Cook until the mixture is set in the middle, either on high for 2 hours or on low for 5 hours. (Cooking longer at a lower temperature will result in a less gooey dessert.)