You can substitute up to half the cream with milk — just use the milk as the cold portion to dissolve the gelatin in Step 1. Note that a half-milk half-cream panna cotta can run the risk of separating as it sets, so you’ll want to whisk the mixture in an ice bath as it cools before pouring into cups, or whisk the mixture when it reaches room temperature before chilling it in the fridge. Here, I thought I’d just keep things simpler and go with full cream.
Alternatively, you could use half-and-half, or for a vegan version, use coconut milk (!)
Find it online: https://tworedbowls.com/2014/05/07/saffron-cardamom-panna-cotta/