My 16 month old is super super picky. We have no "go-to" food, you know the kind that she will eat each and every time I offer it. UGGH, so frustrating. But things that are usually a hit...
She LOVES the salty flavor of all soups. I buy canned soups and strain them to just a little fluid and the rest all the noodles, pasta, rice and soft meats and veggies. Then I add butter and parmasean cheese. or cream. Or DuoCal powder. Sometimes she'll eat a whole bowl and wham that is a ton of calories. But even if she only gets in a few bites, they are some seriously high cal bites!!!
Peanut butter on toast or even just on a spoon.
Grilled cheese, lots of butter and cheese.
Macaroni and cheese.
Ice cream is a big hit around here, and I buy the high test kind that is all heavy cream and sugar.
Good luck, its not fun to deal with a picky eater that can't gain weight. I'm right there with you!!!