I can eat cheese with no problem, and one of my favorite quick high-fat snacks is to spread unsalted butter on a slice/hunk of cheese. Another is to crack a couple of eggs into a bowl, whisk in a bit of heavy whipping cream, and float a hunk of butter on the surface. Nuke for 1:30-1:45, add salt and pepper, eat. The butter blends with the eggs and is divine.
As others have mentioned, scrambled eggs take up huge amounts of butter if you let them, and only get better for it.
I also slice up steak, quickly pan-sear it in a cast iron skillet, then eat it from a bowl with added butter and herbs. Dredge the meat in the butter, then drink the combined meat juice, herbs, and butter left in the bowl. Fast and amazing.
I also use tomato-based Italian sauces over meat, veg, or roasted potato wedges. When heating them I add a massive amount of butter and extra garlic/herbs. It really enriches the flavor.
Speaking of potatoes, and since you want to gain weight, consider adding more carbs alongside the fat. Baked potatoes will hold a huge amount of melted butter, or you could make super-buttery mashed potatoes. I also love occasional steamed yams mashed with lots of butter and a tiny drizzle of maple syrup whenever I get odd pancake cravings (which are really just for the taste of maple syrup, I've decided). I can't do that very often as the carbs completely stall my weight loss, but for weight gain? They might help.