This a Moonmelon, scientifically knows as asidus. This fruit grows in some parts of Japan, and is known for its vibrant blue color. What you probably don’t know about this fruit is that it can switch flavors after you eat it. Everything sour will taste sweet, everything salty will taste bitter, and it gives water a strong orange-like taste. It's also very expensive...costing about ￥16000 JPY (which is about 200 dollars).
Step 1: Look at the directions on the cake mix, Step 2: Add one more egg (or add 2 if you want it to be very rich), Step 3: Use melted butter instead of oil and double the amount, Step 4: Instead of water, use milk. Step 5: Mix well and bake for the time recommended on the box. I did this and it's great - but I didn't double the butter, that would have been too much!