I'll add my voice to the testimonial chorus (if you have a search in the forums for heavy cream you'll find it's a full choir) - heavy cream/double cream, which I mix about 50/50 with chocolate milk, has made a huge difference in my gaining over the years.
That being said, it only really works its magic when I average at least half a pint/500ml for several days straight. Frankly, it's an exhausting experience that I haven't always been able to keep up with. However, if you've got the time to zone yourself out with fattening up, it's an experience I would highly recommend.
ProTip: Mix 1 pint of whipping cream (or single/double cream mix) to a box of plain chocolate brownie mix. Add milk until you get it to get a reasonably drinkable consistency. Voila! 10,000 calorie shake to consume throughout the day
That'd be one hell of a holiday, amiright?! Lol
Best of luck and happy fattening!