Gaining

As far as I know and tested there isn't any type of food or way to gain a soft fat. It should be down to your genitcs. Have a look at your family and the same sex as you. Should/maybe give you an idea on how things will turn on out for you weight gain.

I believe that hard fat is inside the muscle wall, and soft fat is outside of it.

Since you love donuts, try getting a dozen each day and eating them all throughout the course of the day, along with regular meals. You're sure to gain weight. After you reach your goal weight, if the fat is too hard and dense, discontinue the donuts for a couple weeks. This will cause weight loss and the skin that stretched will become softer and flabby. Regulate as needed with the donuts.