I’m not sure why it would be “runny” it’s a thick custard. And once it’s refrigerated it thickens up more. Be sure to cook it to 160°, if you’re done that and try again, add a tablespoon or two of dry milk to the heavy cream mixture as you whip it, that will rehydrate the milk reducing the liquid from the cream and making it thicker.
be sure you are not stirring the mixer at the end it will deflate and become runny, you need to gently FOLD the mixture I’m, very slow, just one fold of mixture then turn the bowl and repeat until it’s all incorporated, but don’t stir it, just keep folding it gently. That is where most people lose patients and stir, and then it will collapse.