01 - Combine butter and brown sugar in a microwave-safe mug and microwave for 20 seconds until melted.
02 - Set the pineapple ring over the melted sugar mixture in the mug.
03 - In a small bowl, blend yellow cake mix with milk until smooth. Pour the batter over the pineapple ring in the mug.
04 - Microwave the mug on high for 1 to 2 minutes until the cake has set. Cool slightly before eating.