What is a good substitute for full cream milk powder?

When I am using skim powdered milk and want more fat, I just replace some of the water in the recipe with heavy cream.

Your recipe doesn't have water, it already uses milk and cream. You could replace a little of the milk with cream to make up the difference.

However, with all the cream and butter in that recipe, I doubt you would be able to notice the fat missing from the dry milk.

I would just use your skim dry milk and not worry about it.

I wouldn't add whole butter if all you're after is some extra fat.

You could try gently rendering down some unsalted butter to get the fat you want.

Do this on very low heat, do not let it burn, take it on and off the heat periodically until it barely begins to foam, then skim off the fat with a small strainer or a tea ball.

Then, save the clarified butter for something else.

A small quantity of that stuff goes pretty far, so you'd have to do some experimenting. A stingy teaspoon per cup of skim milk powder sounds like a reasonable starting point, but try a small batch first for testing.