Stir together 1/4 cup of butter (half of the amount listed), brown sugar and drained pineapple.
Spread this on bottom of slow cooker.
Stir the dry cake mix, remaining 1/4 melted butter and pineapple juice in a large bowl.
Pour cake batter over top of pineapple in slow cooker.
Bake on high for 2-3 hours or low for 3-4 hours.