Preheat your oven to 350°F (175°C). Grab a 9x13 inch baking dish and give it a quick greasing or spray with non-stick.
Dump the crushed pineapple (juice and all) into the dish. Spread it out evenly.
Spoon in the cherry pie filling over the pineapple. Give it a gentle swirl if you like a bit of a marble effect.
Sprinkle the entire box of yellow cake mix evenly over the top. Don’t stir!
Lay the butter slices across the surface, covering as much of the cake mix as you can.
Pop it in the oven and bake for 50-55 minutes, or until the top is golden and the edges are bubbling.
Let it cool a bit before serving. It’s delicious warm, especially with a scoop of vanilla ice cream.