Preheat oven to 350.
Grease 9x13 cake pan.
Lightly drain the pineapple, but leave some juice on it. Pour pineapple in bottom of pan and spread evenly.
Sprinkle dry cake mix over pineapple.
Slice butter in pats and lay them on top of dry cake mix.
Bake 35-40 minutes.
Serve with ice cream or whipped cream, if desired.