Depends on how healthy you want this 'treat' to be...
Personally i'd only look at hard gaining using complex, starchy carbs. Personal favourite is a massive bowl of rolled oats with mixed forest berries or a banana, water or milk, and a tsp of peanut butter.
If you want recipes for a shake to put on some serious mass without all the shitty simple sugars they normally chuck in to those pre-made 'hard gainer' shakes then there's loads on this site but here's one you can play around with:
50gr whey or casein powder
100gr raw rolloed oats
2 tbls natural peanut butter
half a cup natural yogurt
1 large banana
water or milk to taste
mix in blender
Overall kcals should be about 900-1000 depending on whether you add water or milk