Whipped cream should have a cool temperature when consumed (not warm) taking the item with the whipped cream out of the fridge an hour before serving is best. That was the cake portion will not be ice cold but the frosting won’t be warm. If you need more time out of the fridge, make and frost right before serving.

When making whipped cream, the heavy cream will double in size and whip up and thicken. Be sure your heavy cream is cold, you can chill the bowl and whisk attachment. The printable instructions are in the post, here is the link if you missed it. https://www.createdby-diane.com/easyrecipe-print/23890-0/
The dry milk will absorb the excess moisture as it whips and it will become thick and creamy.

I buy dry milk, available in the baking aisle. It usually comes in a box, bag or can. Carnation is one brand, but that will depend on where you live and store you shop at. Any dry milk will work.
Here is a link to one variety on Amazon, but I’m sure it’ll cost less in a local store.Any variety, non fat, whole milk.
https://amzn.to/3FRPypg

I’m not exactly sure if it’ll work out the same, it’s a powdered drink mix that you add milk to, so it’s not actually dry milk. But if that is what you have on hand give it a try. I don’t know if it will hold up as well if you’re using it on a cake, but if you’re eating it soon after making it should be ok.