Can I use my HSA to pay for prescribed donor breast milk for my child?

Generally speaking, you can use your HSA to pay for your breast feeding supplies (pumps, storage bags, etc). You should check with your HSA administrator to determine if the breast milk itself can be purchased with your HSA. 

I'm assuming congratulations are in order! 

Generally, yes. But I say "generally" because it depends on if your specific HSA plan covers that expense. The only way to know is to check with the plan administrator.