Butter a 9" round or 8" square baking pan very well.
Trace the bottom of the pan on a piece of parchment. Cut out and place in bottom of pan - butter the parchment.
In a small bowl beat the butter and brown sugar until fluffy. Using an off-set spatula spread the brown sugar mixture evenly on the bottom of the pan.
Place the pineapple slices in a single layer on the brown sugar mixture.
Spread the cake batter on top. Place the cake pan on a baking sheet as the brown sugar mixture may bubble over while baking (saves a oven clean-up)
Bake 30-35 minutes until golden brown and the top springs back.
Cool in pan on a cooling rack at least 20 minutes.Run a knife around the outside of the cake before inverting onto serving plate. Peel the parchment off slowly so that the caramel mixture releases evenly.
Recipe by Chateau Elma at https://www.chateauelma.com/pineapple-upside-down-cake