Consume more calories than you need, and enrich everything if you can. Butter is the top of the caloric ratio nutrition pyramic I stumbled across, good quality butter can be added to almost everything and improves it.
Nuts are so calorie dense, that 100 grams of nuts can add 700 nutrient dense calories to your days total, and have made everything else easier to digest.
If you wanted a bag of mini cheddars, have a bag spread with butter, and a lump of cheese on the side and a lump of chutney and a chunk of cheese while you make it…..500g of good quality butter is about 3500 calories which you will definitely enjoy, for 1.70.
Milk. 2litres of whole milk is over 1400 calories and yet is actually low in fat.
If you make pancakes, add egg yolks. Eggs and fat, hollandaise, aoili, egg mashed up with butter, omelette with extra melted butter…really that tray of eggs can be combined with fat in the most glorious ways. Have butter on your toast.
Olive oil, if you are having pasta, toss it in olive oil before it even hits the sauce…sneak those calories in.
Learn to bake, eat everything you ruin and everything you don’t. Make brownies, this brownie recipe requires a pint of chocolate/egg/melted butter to be added to the dry ingredients. Lemon cheesecake is lightest way to consume cream cheese and even in the worst states I have to make two, because one is not allowed to set.
Take your time perfecting your sauces if you are at that stage of culinary skill. Sauces are a very good way of making pourable calories to disguise the taste of dull food.