You can mix the butter cold from the fridge when using a stand mixer, in a hand mixer it’s easier if the butter is a closer to room temperature butter (about 65 degrees) cut the butter into small cubes it’ll be easier to mix and will come to a room temperature quicker. Either cold or room temperature butter will work in this recipe, I’ve use both with great results.

Great recipe for cut-out cookies. I’ll definitely make it again!

When you say they didn’t hold their shape, specifically the dough was too soft and it it flopped when you picked it up with a spatula. Was the dough rolled to 1/4”. Was the butter warm above 67 degrees? Soft butter can be a problem. As mentioned in recipe add a bit more flour as needed so the co ok ie dough is not too soft. Maybe mix another minute or two, add 1/4 cup flour and remix when The cookie dough is in a bowl and you take your index finger and you press into it. It should be firm enough that it resist your finger from going all the way to the counter. If when you press it it feels like it sinks super fast makes it a little longer. Add a little more flower until the dough is firm enough It’s hard to find out what exactly is the problem Without more detail details. And not knowing how much experience someone has with cut out cookies, a little more detail would help.