Ah, substituting cream with milk and butter... Reminds me of the time I tried improvising a white wine and garlic sauce for a garden party when the local shop ran out of dairy. Everyone was tucking into the burgers on the coals, while I was inside frantically Googling culinary alchemy! You certainly can melt some butter into milk to up the fat content, but I honestly cannot remember if it's a third of a cup or a half cup of butter per mug of milk... It might work a treat, or it might just separate into a greasy puddle on your pasta. Good luck with the meal prep, anyway!