Can milk be substituted for heavy cream when making butter cream?
I'm not sure exactly what you're talking about when you say butter cream. The butter cream I am familiar with is frosting that uses butter, not heavy cream. Are you making a sauce of some kind?
Usually recipes that ask you to use heavy cream rely on the fat in the cream to thicken properly.
Yes, butter cream frosting, particularly this recipe:
Any liquid can be substituted as long as you don't add too much, it will break your frosting.
I like to use buttermilk to cut the sweetness..
Also good is using marachino cherry juice to make a cherry frosting, flavored coffee creamers to make whatever flavor the creamer is (just adds a hint of flavor, a little some'n some'n), lemon juice, orange juice, etc.