Preheat your oven to 350 degrees F.
Put the butter in a 9x13 pan and put in the oven just long enough to melt.
Remove the pan and distribute the melted butter evenly across the pan.
Add the brown sugar to the butter and stir through evenly.
Arrange the pineapple and cherries evenly on the brown sugar. If you have pineapple rings place the cherries inside the rings.
Combine the flour, sugar, baking powder and salt if making your own cake mix.
Combine the eggs, oil, and reserved juice together, and then blend with the cake mix to make the batter.
Pour the batter over the fruit in the pan, it should level itself.
Bake for 40 minutes, or until a toothpick inserted in the center comes out clean.
Let cool for 5-10 minutes (don’t wait longer, the sugar may stick to the pan) then run a knife around the outer edge to loosen it and place a large platter over the top of the pan. Flip them over together and the cake will release onto the platter.