as does the same amount of any other substance, including for example yoghurt.

That number is just the E in E=mc²

If we are actually talking dietary calories, the number you see on foodstuff packaging, plutonium has 0.

It would certainly feed you for the rest of your life.

Unfortunately, if one were to invest a single gram of plutonium, not only are you ill equipped to actually properly harness even a small percentage of the heat energy it will produce, it also qualifies as a heavy metal, so there is every probability that whatever you do actually ingest before the chunk of plutonium exits your rear end will end up in your bones, thereby irradiating you further than the original gram of plutonium irradiated you as it passed through your digestive tract, mostly unobstructed, provided one doesn't have a previous bowel obstruction.

All of that is to say that it probably tastes spicy metallic. Especially if you have fillings in your mouth. It's energetic enough to give a similar sensation in your teeth that a piece of aluminum foil does when it touches your fillings.