Guidance for most sedentary American adults (ages 31-50 [1]) is to consume 1800 kcal a day. 100 kcal is 5.5% of your daily caloric budget. You just consumed 5.5% of your daily caloric budget with that one extra tablespoon of butter.
Assuming you have 3 meals a day, and assuming you have 600 kcals per meal, and assuming your nutritionist has prescribed a diet with a 50/20/30 carb/fat/protein ratio, and assuming one of those meals (dinner?) has 100 kcals from fat, that means 16% of the meal was fat. If you eat a lean protein (with a tiny amount of incidental fat) and carbs with that meal, that's actually the perfect amount of fat.
You can also mix up your meals so that, say, breakfast has very little fat, and dinner has more fat, or vice versa. It's all about balance.
[1]: https://health.clevelandclinic.org/how-many-calories-a-day-s...