Butter, olive oil, bacon, chicken skin. Yum!
A common misunderstanding that I often encounter is the belief that eating fat makes you fat. This is only situationally true.
For fat to be truly fattening, it needs to be combined with carbohydrates. When carbohydrates are consumed, they trigger the release of insulin from the pancreas.
Insulin is the hormone of STORAGE.
Once insulin levels rise, both carbs and fat are escorted into cells for storage.
Consider chocolate chip cookies, ice cream, mocha lattes, chips and guacamole, and french fries. These are all examples of high-fat and high-carbohydrate foods combined--Very FATTENING foods.
On the other hand, if you consume just butter (or other mostly fat foods), insulin is not significantly increased. As a result, those calories are not shuttled into cells for storage for a later date. So, they will not make you fat.
The key to determining whether fat is truly fattening lies in controlling the hormone insulin. You can control insulin by monitoring your carbohydrate consumption.
So, fats aren't inherently fattening unless you add some sweetness (sugar) or crunch (chips).