Place butter in a 9x13-inch baking dish and put pan in the oven while it preheats to 350 degrees F.
Once butter has melted, carefully remove pan from the oven and sprinkle brown sugar replacement on top in an even layer. Arrange pineapple slices on top and place cherries in the center of each pineapple slice.
In a large mixing bowl, combine cake mix, protein powder, egg, yogurt and pineapple juice (add enough water to reserved pineapple juice to measure 1 cup).
Spread batter over pineapple in an even layer and bake for 38-40 minutes or until lightly golden brown or until an inserted toothpick comes out clean. Immediately run a knife around the edge of the pan to loosen cake.
Place serving plate upside down onto pan; turn plate and pan over and leave inverted for a few minutes until all the topping leaves the pan; remove pan. Serve warm or cold but store cake covered in the refrigerator.